2.5 is probably what made TSR worth buying.

That aside, it might also have to do with the fact that people get genre- and system- tired. DMs can only run the same shit so long, and most editions last many years. Even 4e lasted longer than most people think it did, these days.

What happened in the 90s was White Wolf, Call of Cthulhu on the Horror end along with 40k and Star Wars on the battlemap end. What happened in the 00s was a huge surge not just of D&D adjacent games, but things like FATE and DW and BESM and a number of other new systems that tried to do away with the roll-to-resolve-hits approach to things.

And now we have an entire generation that's played nothing but 5e. Whatever the straw was that broke the camel's back, there was also some writing on the wall that there was going to be a pendulum swing.

After each of those failures though, they came back and quintupled their audience. So there's that, too.