I would imagine because Gravity is not seen to be in the top tier of Control Sets. I like(d) my Gravity Troller, but YMMV. I remember being very annoyed by the lack of damage early in my hero-ing (some of the angst was due to me being a newb and unaware of what exactly a controller was). Singularity (the pet) and Wormhole (a teleporting AoE disorient) make the set as far as I'm concerned... not to mention the fact that chucking debris everywhere is fun too (that would be Propel). Unfortunately, the coolest powers come late in the Grav Troller's career. 
I paired mine with Force Field, I found this to be a bit too passive and so grabbed the Medicine pool so that I have a bit more to do during combat. I have seen it paired with Storm (watch out though, as the Immobs in Grav do not have -KB, the holds do) and Rad as well, to great effect.
I'm a total noob and am running with a Controller Grav/Storm. I tried two different blaster combos before trying out the controller and while the low dmg is a pita sometimes I've found myself taking on 4 orange mobs at a time and doing pretty well. This without taking Propel (yet). Just make sure to grab yourself Hover to get out of the way of people and you should be golden. Whatever you do, make sure you don't lose track of your stamina or mobs. CC keeps you alive and stamina keeps you flying out of reach of the heavy hits. 
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I don't see a whole lot of Gravity controllers and I have never attempted it. I don't last long with controllers. I tend to lean towards blasters and scrappers. But I'm thinking of rolling one out but I'd like to know what I can expect from it. And is there a reason that I don't see it being used often?
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Because until about level 27-29, when you have a slotted Wormhole it limps, with little area control.
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If I were to choose it what would be a nice secondary to compliment it?
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/Storm and /TA go well with Grav, making up for Singularity's lack of damage. Not that Singularity is bad; it's a great pet, just not a damage pet. More of a Controller/Tank.
Arc #40529 : The Furies of the Earth
I like gravity's movespeed debuffs. But most people don't give a damn.
You might give Grav/Storm a try.
Grav is my least favorite controller primary, and the only one I have yet to get over 32. My highest Grav Controller is only in the low 20's, and I have a Grav/Rad and two Grav/Storms on different servers. Why am I not so crazy about Grav?
(a) lack of AoE control until Wormhole at 26 -- you only have an AoE immobilize and a slow-recharging AoE Hold -- and even Wormhole is on the funky side.
(b) lack of interesting powers. I find three powers in Grav to be interesting enough to make it stand out -- Propel, Wormhole and Singularity. The rest of the powers are unremarkable and pretty boring.
(b)(1) and as visually cool as Propel is, it is very frustrating on teams. It feels as though most of the time that I try to use Propel, by the time it finishes animating, the target has already been defeated by a teammate. The animation is so slow that many Grav controllers skip it, even though it does very good damage.
(c) Before Wormhole, you have almost nothing to use as a "panic button" power to save your tail. Even solo, I find my Grav controllers take a lot of damage. This is, in part, due to the slow animation of Propel and because the main AoE is an Immobilize. One of the things I was most looking forward to was the combination of Freezing Rain and Crushing Field, since Crushing Field is a unique AoE immobilize without -knockback. But the knockdown on Freezing Rain is not all that dependable, so the combo is just not that impressive -- I still take a lot of damage.
Fire and Earth have AoE Stuns at level 12. Ice has an AoE knockdown at 12. Illusion has Deceive early and Phantom Army at 18. Mind gets Confuse early, an AoE Sleep and excellent single target control/damage powers in the lower levels. Grav doesn't get much of anything really good until 26.
I can see how Wormhole and Singularity could be a lot of fun, so I plan on continuing to pick up a level or two every now and then on my Grav/Storms, but other controllers just seem like more fun.
And I think Storm is the best secondary for Grav. It gives you another stun to stack with Wormhole, great positioning tools and damage in the upper levels. But then Rad is great with anything, too.
LOCAL MAN! The most famous hero of all. There are more newspaper stories about me than anyone else. "Local Man wins Medal of Honor." "Local Man opens Animal Shelter." "Local Man Charged with..." (Um, forget about that one.)
Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
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(c) Before Wormhole, you have almost nothing to use as a "panic button" power to save your tail.
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Actually gravity has a great panic power... however, it just phase shifts everything and doesn't let you kill anything. But "panic-button"-wise, it's great. 
"Honesty is for the most part less profitable than dishonesty." -- Plato
Playing Gods (51106) - Heroic Lvl 5-20
What Rough Beast (255143) - Villainous Lvl 40-50
Thank you for the wonderful responses.
Looking at it, I was worried that it would be a slow powerset. I have a feeling this isn't going to be for me, but I think I'm going to try it anyways. I could surprise myself. The idea of Gravity interests me so hopefully that will be enough to carry me along.
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(c) Before Wormhole, you have almost nothing to use as a "panic button" power to save your tail.
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Actually gravity has a great panic power... however, it just phase shifts everything and doesn't let you kill anything. But "panic-button"-wise, it's great.
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Whoops, you are correct. I forget about Phase Shift powers because I never take them (and very few people take them, either).
Anyway, to the OP, by all means, try Grav and see if you like it. There are many folks out there who love it. And even a bad controller is better than most other kinds of characters . . .
LOCAL MAN! The most famous hero of all. There are more newspaper stories about me than anyone else. "Local Man wins Medal of Honor." "Local Man opens Animal Shelter." "Local Man Charged with..." (Um, forget about that one.)
Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
My Grav/Kin is still my favorite character, mainly for thematic reasons. It's an old combo, I know, but I find the two go well together. Grav is still one of the strongest single-target control sets in the game, and as my Grav/Kin is based on a teleporter theme, the Grav set is a must.
Of course, knowing your character could destroy the world (or even the solar system) if he/she gets carried away can make some people hesitant about playing the powerset
From a game standpoint, Kinetics is a great team-support companion to Grav's single-target emphasis, and Singularity is probably the single most effective companion pet to a player character in the game, effectively more than doubling his/her capabilities.
My best advice, as I give to everyone who asks, is to play what appeals to you. Don't grab Grav expecting the 'I-win' powerset, or because it will dazzle everyone around you...it won't. Grav is for players who -want- to throw things around without touching them, and take down one opponent at a time. If you want bling powers or shock-and-awe firepower, you'll probably be happier with another powerset or archtype.
Those who think Truth is relative haven't had a Tank land on their car.
I don't see a whole lot of Gravity controllers and I have never attempted it. I don't last long with controllers. I tend to lean towards blasters and scrappers. But I'm thinking of rolling one out but I'd like to know what I can expect from it. And is there a reason that I don't see it being used often?
If I were to choose it what would be a nice secondary to compliment it?