This is very true, I believe that Damage Resistance isn't in there either. It is in training, it is in single origin. Just not in dual.
In SO the endurance ones have another name as well, they're right under Recharge Reduction in the list, but can't get the name out at the moment.
It's been like that for a looong time now.. Well, since MA basically 
So I'm plodding along on my Dark/Stone tanker, and having finally finished off all the 15-19 content, I turn XP back on, beat up a couple of random mooks that cross my path, and woohoo! It's Stamina time.
Rather than faff on alting to my badge toon and making up a level 20 End Mod IO, I decide to try and be moderately cunning. I go to the Kings Row AE building and pop into a single mission arc, wander through it and exit with 100 tickets in my pocket.
Off to the ticket vendor I pop, only to find that apparently, while the ticket vendor stocks Training Origin End Mod Enhancements, they appear to be completely out of Dual Origin Endurance Modifications...
Whuh?
In fact, looking at the list, while 26 flavours of TO are available, only 19 flavours of DO appear to be available. I hope this isn't some mutant child of the 'Power ten' concept coming back again...
Warning:
The above post may contain Cynicism, sarcasm and/or pessimism. If you object to the quantities contained, then tough.