The Behemoth released the game for PS3 in 2010, but they never announced or worked on a PSP port.
Castle Crashers, developed by The Behemoth, is a game that wears its influences on its sleeve. With nods to classic arcade beat-em-ups like Teenage Mutant Ninja Turtles and side-scrollers like Contra, the game is a loving tribute to the classics. But don't let its retro aesthetic fool you - Castle Crashers is a game that has aged remarkably well, with tight controls, colorful graphics, and a healthy dose of humor.
Notice the gap. The PSP was effectively dead by 2012 in Western markets. Sony had shifted focus to the PS Vita. Developing a backward-compatible PSP ISO in 2010-2011 would have been financially pointless for The Behemoth.