GT5

was a glorified demo, and if you paid $40 for that... I lol @ u.
Anyway... PD's history in the time span of GT5's production is somewhat documented here on GTP. They had a lot of things going at the time which definitely put GT5 on the back burner for a while.
Tourist Trohpy,
GT

SP,
GT5

with active online and updates/DLC.
It probably didn't help either that they had to completely code GT5 from scratch for the PS3, which as I understand is more difficult to program for because of the multi cell processor.
And of course GT5 also has
a lot of features that no other GT title has, and to venture into those frontiers probably did take some time to R&D. Though the results are pretty lackluster, I'm sure they've gone through countless tests to find what will work for the user, for the game itself, and for the PS3. That's a lot to balance IMO.
Then you have other projects on the side that have PD's stamp on it, like the digital interface for the Nissan GTR. And as I understand it, they do side projects for automobile companies all the time.
If this wasn't enough, Kaz had embarked on a bit of a racing career himself. And they had the whole GT Academy to oversee...
I can definitely see how it took 5 years to finally release GT5. Small company probably spreading themselves too thin to juggle all this business. They did the best they can and they're rapidly fixing and giving due attention to anything in GT5 that isn't satisfactory.