because the 250r was god of its time.. its frame geomitry, and suspension were ahead of there time. HUGE aftermerket back then and a even bigger after market for them now.
they were the most reliable of the big 3 250 2 strokes.
plus they had a good start and a head start in the sport/ race atv market... the first 'race' atv was the 81 honda 250r atc 3 wheeler. which made its maden run in the 1980 baja as a pre production run.. so they had a 304 year jump on yamaha and suki and kawi. lots more r&d time