Theres many things that factor into purchasing an FPS for me...the biggest one is the online multiplayer competitive/cooperative with friends. If i can't play it in either one of those two, then its a much less likely chance for me to actually care about the game. Having those are huge bonuses to replayability, and for most of my games I like to keep and play them often. Single player can only do so much.
Another is control...I highly prefer a customizable Keyboard/Mouse over the Pad...although thats more of personalization, it just makes more sense to aim with a mouse. So yes its inadequate controls

. The depth of field is pretty important too with the monitor imo, as well as the amount of players that would play a PC FPS compared to a Vita.
Thats just me though...I was always surprised at the fact that there are CoDs in handhelds. But hey if it appeals to some...i dont see why they shouldn't make one.