But that'd be a bullshit excuse. Modern game engines are designed to be convertable and PSV is plenty capable of accepting what's in Activision's toolbag. To get a game as stunning as what we see on PS3, sure, that's hard, but just getting COD's IW engine up and running on PlayStation Vita would not at all have been an impossibility.
But like I said earlier, it's almost like Activision took the hard way out by getting an outside developer involved! Activision has been were laying off staffers at Neversoft and Vicarious Visions, then shutting down Radical and before that Bizarre and 7 Studios, there's tons of talent right there waiting for a project that never came their way. And Activision has an engine that four teams know like the back of their hands. Putting 30 more guys in a corner at Treyarch (or they could have got all those guys who used to have to shove every COD into Wii somehow, they're not doing it this year?) or getting the Raven and Sledgehammer guys to take a couple unfinished COD maps from the pile and fit them into a new game would have been reasonably easy. (*Actually, considering Sledgehammer has been a total no-show as far as the fans can see since COD Adventure died, this could have been a nice opportunity to finally give them a proper calling card.)
Also, never mind that Activision had no idea how many units PSV would or would not sell by the time they finally got around to shipping this game they announced over a year ago. Those stats don't really matter because if the Vita version underperformed, this would have been an easy project to flip back onto PSN/XBLA/Steam (that might be what Ubisoft is counting on as a fallback with AC3 Liberation even though they're clearly managing that budget smartly) and sell as a mid-season COD game release.
Between all that Sony support and the easily reused engine and the fallback of a HD re-release later on and the possibility (however slight given the weak DS performance, though iOS buyers are taking to COD happily) of having another eager market for COD products, it was a win/win/win/win IMO for Activision to put some effort into this Vita game.