There are two "things": the trail (or road) and me. And the bike is the link between these two. The more simple, more reliable link the better.
Nice statement! Yes, like a simple old sportscar - for the purity not the sophistication.
Another reason - though a bit vain maybe - is the sort of "inverse cool" of it. Next month i'm riding a 45km MTB jungle night race / jamboree with 2000 entrants. Where i live nobody rides retro and most guys have not even seen a '90s MTB (the MTB scene hardly existed here at all until the modern MTB explosion a few short years ago) so everyone stands on the startline comparing their fancy hydroformed frames, 29er rims and long travel shock gizmos. Just for the fun of it i'm going to leave my full susser at home and take my '90s steel hardtail with rigid fork and cantis and actually race all the 20-something youngsters on it! I finished in the top third last year on a modern bike so what the hell - should be a gas and hopefully won't come last!