It can all be explained by the radical change in the demographics of computer owners starting from the early 90s. The truth is, only rather well-situated people could afford a PC in the 80s, so quite naturally those were educated, literate people. Can you imagine a lawyer, a dentist, or a scientist playing some DOOM clone for more than five minutes or some kiddie platformer? Naturally, those folks were drawn towards more cerebral games such as strategy games, adventures, and RPGs. Who do you think Sierra sold all those rather expensive Roland MT-32s to? Elementary school kids?
Then prices went down, suddenly everybody had a PC, and you had to sell something to those masses... Guess what, they were not into strategy games and adventures, neither RPGs. But their money had the same worth, and there were a *lot* of them. You can work out the rest.
It's a bit disingenuous to think that Americans = stupid, Easter Europeans = prestigious. The birthplace of AD&D and cRPGS was most definitely the USA, then there were some much smaller scale efforts in Europe with a phase delay throughout the 90s. The "decline" is simply the result of changing market conditions and US people generally being good at the business side of things—more complex and demanding games were no longer a lucrative business after the early 90s. And maybe there's an element of Europeans being perhaps less money-driven in general, so some studios have this "fuck it all; we'll do what we love no matter what" attitude over there a bit more often perhaps.