A few things:
1) /Fire deals Fire damage, while /SS deals Smashing damage
2) /SS has more mitigation in it's attacks than /Fire does. /Fire has as it's mitigation causing death faster, which, while it works, can be somewhat slower for mitigating damage
3) /SS has Rage, which can significantly boost damage output, especially on AoEs when you can leverage having more people to hit with them.
4) /SS tends to be more smashy and fun for some
5) Since Shield Defense has it's own AoE (even on a long recharge timer), it adds another AoE to /SS, which can help it out a decent amount.
6) The combination of Rage and Against All Odds can vastly increase the Tanker's damage ability.
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted.
~Ralph Waldo Emerson
"I was just the one with the most unsolicited sombrero." - Traegus
Thank you sir! That's all I needed, I appreciate the quick response
Just to add, that is not to say that Shield/Fire is a bad combo. It's actually probably pretty good at taking out large spawns pretty quickly. Those were just the reasons why you probably see more about SD/SS than SS/Fire. I'm sure that there are just as many reasons to make an SD/Fire.
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted.
~Ralph Waldo Emerson
"I was just the one with the most unsolicited sombrero." - Traegus
SD/Fire & SD/Ice lets you wield a sword & shield combo as a tank. If you happen to care about things like concept, getting a flaming (or frosty) shield and whacking things w/a flaming (frosty) sword is pretty damn cool. 
An Offensive Guide to Ice Melee
/Fire has more AoEs and generally, much better attacks, but when a character has the IOs to grant himself double stacked Rage, /SS is able to gain slightly better damage while having the recharge to put Foot Stomp on an endless chain of destruction.
Virtue: The-Invictus (Blue)
The Emissary of Justice - Level 50 Fire/Fire Blaster
The Emissary of Justice. - Level 50 WP/Fire Tanker
Mesmerius - Level 50 Psi/Mental Blaster
Nucleoa - Level 50 Rad/Son Defender
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/Fire has more AoEs and generally, much better attacks, but when a character has the IOs to grant himself double stacked Rage, /SS is able to gain slightly better damage while having the recharge to put Foot Stomp on an endless chain of destruction.
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You don't need IO's to double stack Rage.
The Rage mechanic and the huge radius (and fairly short recharge time) of Footstomp is usually what drives people to like SS over Fire in my experience.
Also, the Fire (and Frost) secondaries seem to appeal to less people on a thematic scale.
One thing I haven't seen mentioned is the fact that Foot Stomp has an absolutely huge 20' radius.
Smash, as in none in fiery melee and a lot of of it in ss.
After that, I'd pick ss over fm even if just for rage, footstomp and kob.
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One thing I haven't seen mentioned is the fact that Foot Stomp has an absolutely huge 20' radius.
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Yes but I don't like to brag 
Just a quick question. Trying to decide SD/SS or SD/Fire. I see SS is popular on the forums, haven't seen much on a SD/Fire, but it appears to me you'd get more AoE out of Fire for tanking big packs. I'm pretty new and cant figure out why SD/SS is so popular and I don't see much about the SD/Fire.