There has to be a reason, no? Metal Gear Solid designer Hideo Kojima could have put the game on home consoles, but he chose portables. Here's at least one reason why:
In an interview with Famitsu, Kojima told Assassin's Creed II developer Patrice and Patrice's beard:
Really, I don't think much about whether a platform's portable or for the home. In other words, it's not important to me. With the PS3, you've got a whole range of middle- and high-school kids who can't play your game because their parents don't let them play video games at home. Those kids can whine at their parents all day about how the game's anti-war and anti-nukes, but they'll just respond 'Well, you're still killing people with guns, aren't you?' and brush it off. I wanted to go the extra mile for gamers in that sort of situation. That was one of the reasons I chose the PSP.
Kojima isn't doing it for me or you, he's doing it for the kids. And the kids are alright. Elsewhere in the Famitsu piece, Kojima tells Patrice what games stand out in late 2009 (Modern Warfare 2, Uncharted 2 and Assassin's Creed II) as well as talking about Assassin's Creed II improvements. Patrice gives his impressions of Peace Walker. Sadly, Patrice's beard does not.
Metal Gear, Assassin's Creed Creators Interview Each Other [1up]