I honestly think it's just like every other online game I've ever known... You have to socially network either here on GTP &/or on PSN itself, to find the kind of people who like to play the way you do, and then find them again & again.
It's been this way for every online game I've played for over a decade.
Not to negate Johnnypenso's various points of course, all very valid & legitimately contributory to the situation.
But I'd like to take a different viewpoint and say that while super cars & drag racing makes positively no sense to me at all... I imagine some people are enjoying themselves or they wouldn't be doing it.
And I think the nature of gt5 is such that it does tend to target to appeal to a VARIETY of people with a variety of interests... from collectors who have to have every car in every colour... to people who don't bother with paint chips or replays, they just want to get on and race whatever...
I think we take the good w/ the bad w/ gt5... in that if it didn't appeal to a wide enough audience, they'd never make the money to make the game as good as it is. It has problems, many... but it really is an amazing game for what it is, which is why we all like it.
I don't even know what a JDM/Stance Meet is!!! ???????
But there ARE many of us who like racing... proper kind of racing of one kind or another, in the many road cars gt5 offers.

And for things other than drag or drift.
It's not dead.
But lots of people are outside in their free time, where much of the population that plays this game, it's the summer season.
And you know, it's been awhile since I've raced a lot, since I spent most of spring & early summer tending to my gardens. Spending lots of time outside, and then being too tired to organize races or even to race.

(Or with family or friends visiting or general outings & such.)
I mention this because I see your GTP join date as : Jun 07 2012
It's really hopping with the more mature folks who race when it's dead of winter in Europe, Northeast US, & Canada, getting dark by dinner time, and too cold to be sitting outside or spending hours on the lake fishing after work.

(Or mowing the lawn. heh.)