Re:
Three good reasons it took so long for discs to become popular...
Weight, expense and complexity. Probably in that order.
The good thing about the rapid growth of the mountain bike market throughout the 90's meant that discs became a viable option as the benefits began to outweigh the negatives.
Hope did a great job, at least in the UK, of persuading many riders of the benefits of discs.
Practically everyone I know started with Hope as there first foray into disc brakes from about 1996 onwards.
As with most 'new' things in cycling, they aren't actually new but either previously unpopular or just obscure.