Well, I guess there are more then few reasons for delaying.
First, launching GT game for PSP without enough consoles in installed-base is much-less a waiste - having it done when base reaches like 20 mills will ensure instant sale of millions of copies. So far, 10 mills PSP's has been shipped worldwide, and Sony stressed that they expect another 10M in the end of 2005 holliday season.
Also, Sony is probably "protecting" other racing titles for being over-saled by GT, ensuring them a nice piece of income in the first period of PSP's "life" - once when GT Mobile reach the shelves, I guess no other PSP racing game will be taken seriousely.
Third, PD has a lots of other jobs to do. As a relatively small developer (as I know, less then 100 people work in PD), theyhave to ensure propper allocation of workflow - some people have been working on Tourist Trophy (out on 31st December in Japan), while majotiry of stuff is probably very busy with next-gen GT game. Which does not leave them much space for PSP game.
It's worth to mention that so-called "GT Mobile" has been changed in process, due to some Kaz's statement in last 12 month - in the first announcements, "GT Mobile" should have been the same game as GT4. Lately, he stated they've changed the "nature and focus" of the project, focusing it on multiplayer aspects and making some changes in gameplay in compare to existising GT games. Some sources even speculated that PSP GT will be some kind of crossover product between GT and something that Kaz reffered to as "GT For Boys" before the Japnese release of GT4 in December of 2004.
However, after those information no word came out about "GT Mobile" and this is the first news in last 6 months about this very game.