Based on your tags I am assuming you are talking about the version that hit 360?
The honest answer would be the window of opportunity just never was there. Back when bully hit 360, PS3 wasn't popular. It was alot harder to develop for and the price point was way above what most would pay. So they put the game on the two viable systems at the time PS2 and 360.
I just don't see there being enough of a niche market to warrant porting it over at this point to PS3. There are publishers out there like Atlus that make money off smaller publishing distributions of games, but making sure all the legal stuff is in order and paying the cost of porting VS how much the game will actually make and I just don't see the money.
That is most likely the reason why. Luckily rockstar will most likely revisit this franchise when they have new creative ideas to add to that series.