Well, at some point, todays new bikes will be retro.......so I guess that it is only natural that the late 90's are being considered retro by more people.
However, nothing can change the fact that the prime period of offroad cycles was in the eraly to mid 90's. The bikes was still using rigid forks for the most part and hand made works of art like Klein and Fat (and many more I guess) was in it's prime.
Ringle, Paul, Kooka and many others.....they all peaked in this period and high end Shimano equipment still had it's extreme finish and quality.
Also take into account that this period was at the peak of the biking trend where "everyone" was converting to offroad cycles and using them for transportaion instead of cars and/or public transports. Today, my impression is that modern bikes are mostly used for working out and at best commuting.
This will not change and with the ecxeption of SS i havent seen any similar trend since the mid 90's.