I sometimes wonder if GTPSP is the fastest GT to die. Heck, even the GT1/2 forums get more life than this section sometimes.
I think the reason it came and went was because we're not accustomed to a mobile version of GT. The concept is just too far removed from what we're used to with GT1/2/3/4, and it doesn't give us the same satisfaction as previous GT games. Those of us that can accept it played it just long enough to beat it or collect all cars, then move on. For those that are completely new to the world of GT, or only have a PSP, might have enjoyed it because it is one of the best racing experience on a handheld. However, I get the impression that PSP players don't visit forums much (?), so this forum gets deserted pretty quickly once the 'established' members are done with the game.
I remember Kaz once said he intended GTPSP to be a 'gateway' for new gamers to the world of GT. It's sort of an introduction to the more serious GT on home consoles. In that respect, I think it has achieved its goals.
For me, I treat it as a For-When-I-Don't-Have-My-PS3 game. I rarely played GTPSP at home anymore now, aside from the occasional blast at the 'Ring, but when I'm going out of town for a few days (so it's impractical for me to bring my PS3), GTPSP is my saviour from boredom.
No matter how you look at it though, this iteration of GT won't be a full experience. The way I look at it, there are 2 hints PD put in (or rather, left out) that indicates they don't view GTPSP as a complete GT: silver GT logo during loading (not blue/red) and lack of Moon Over The Castle anywhere in the game in all versions (the short bit at then end of the GT5 trailer doesn't count).
So yeah...it does came and went. But it was designed that way (sort of).