Licencing issues. It is possible that the original game could make it up some day, but to get GT2 up Sony would have to jump through so many hoops to get together all of the licences that they had when the game first came out that they probably wouldn't bother with it.
As one example, the Vector licence that PD had gotten for the M12 and W8 in GT2 is something that probably could never be replicated again, due to the messy legal battle that the company went through almost immediately after the game came out.