Doesn't matter whether it's millenials getting their kicks from video games and not being interested in real vehicles (although I do know plenty that are), increasing cost and complexity, the general inaccessability of motor sport or performance machines being less relevant than vehicles as fashion accessories due to increasing road rules and regulations. In the end it all comes down to less interest and the end of "win on Sunday sell on Monday". Even though they tend to blame the current dominance on the sheer amount of money and engineering needed to be competitive with the current machines and electronic rider aids, MotoGP has always been dominated by elite talent (riders, not drivers). I started watching when Mick Doohan would just clear off, and the competition was largely for second place. Winning on a bike that wasn't a Factory Honda would be a big ask indeed. Much the same situation with Valentino Rossi (which made his switch to Yamaha all the more impressive). Even back in the days of Giacomo Agostini that was true. The outright winner wasn't as important as the racing though, and watching ludicrous sideways antics from riders like Gary McCoy was always great no matter how (un)competitive his bike was or where he placed. Likewise WorldSBK rules heavily favoured the Ducatis until very recently. Even when Fogarty was utterly dominant on the Ducati, watching Noriyuki Haga taking unconventional lines sideways on a supposedly uncompetitive YZF750 was something worth watching. Locally the relatively cheap historic/classic racing and super moto racing events still tend to be dominated by "retired" Superbike and MotoGP riders. Never saw the appeal of Nascar or speedway (i've been 2-wheels only for decades now) for the same reason I think races like Super Stock, Hill climbs, the Isle of Man TT and North West 200 are the ultimate. The machinery, skills, course and riding are relevant to the bikes I ride and where I ride them.