For me its just fond memories of the cool gear when my mtb obessions begun (93-98ish) - although I like new stuff too.
My most recent ride is a 97 Giant ATX 980 so I'm kind of stuck in a past era by default. Perhaps not the most retro, but to me it was a cool era in the evolution of mass market mountain bikes (acceptance of full suspension for general trail riding, emergence of v-brakes and decent forks below pro level etc).
Also agree that the chance to acquire gear you always wanted is a factor too, although I wouldn't do a full retro build. Love looking at them, and have heaps of respective for those that put the effort into restoring them, but the chance of improving on the original is too much for me - in the process of building up an old GT STS with new kit now...
1997 Giant ATX 980
2007 On-One Inbred