Sony holds a contract with Rockstar for Exclusivity with there consoles. Meaning, they contacted Rockstar and gave them a lump of cash to specifically develop the game only for Sony branded Consoles, mainly the PSP. It was done so Sony could finally get a "Killer Game" on the PSP to compete with other handhelds better. The success of the game led to Rockstar porting both stories titles to PS2. It is unknown if they will ever make a another Stories title or port them to newer consoles. But either way, they will always be Sony Exclusive.
Similar to how Nintendo has a contract with capcom to keep Resident Evil Zero and Resident evil Remake exclusive to Nintendo consoles.
One thing they never did was conclude the stories set since there was no San Andreas Stories. This could of been set two years before the events of San Andreas and Rockstar could of used this to answer some questions as to what happend before the events of GTA San Andreas and after the events of Vice City. They could of also answered a bunch of other questions in the actuall game if San Andreas Stories ever got made. Sadly people who bought the Stories titles are left with only two games out of a possible three. Also it would of given people something to do while waiting years for GTA IV at the time.
With todays advancements with consoles and the technology Rockstar could treat us with two new GTA titles and make San Andreas Stories a digital download for those on Xbox 360/PS3 etc or perhaps even on PS4/Xbox One. It would give GTA Fans enough to do.
By the way does anyone know why GTA 3,Vice City and San Andreas has not been remade in HD for the Xbox 360/PS3? Seems odd they wouldnt celebrate the 10th aniversary by doing this and they would of made money doing this to boot.
Phew rant over thanks for reading