Principate
Saint Titanfall
Even in the laziest of situations the bottom screen of the 3DS is used for actual useful things like HUDs, maps, or quick access to menus where most lazy developers will just ignore the back touchscreen on the Vita.
It's not a visualised gimmick so of course no one would care. When you see the Vita the first thing you notice isn't that it has a touch pad on the back, it's the very last thing you notice, if you notice it at all. Even if it's pointed out, the most a customer can really say is "that's nice...."
It's an extremely unnoticeable gimmick, which even worse is that doesn't immediately make the cusomer think of ways it could be used to improve software. Add to this that Sony doesn't even properly show these worthwhile ways, and you end up with a gimmick a developer doesn't care about and a customer even less.