I think that if Liberty City Stories and Vice City Stories were also made available for PC players (and possibly XBox players), that would have greatly increased the chances of a San Andreas Stories.
Like Azurex said above, the scale of San Andreas would have been way too much for the PSP to handle, even if the game was made with the quality of Advance.
My idea for a San Andreas Stories is actually fully fleshed out (I will not post it on these forums, however), and it centers on Mike Toreno and the rise of the Loco Syndicate. Toreno is sent to San Fierro on a government job,
his superior agent is the main antagonist, and Toreno's operations coincidentally help those of his future Loco Syndicate allies, leading to the unlikely but fruitful friendships.