in the last 20 years there have been a handful of legit superstars in track.
track and field level of talent is mediocre overall.
in the distances, the talent has gone to the roads, marathon.
great talent goes into soccer, other pro sports which have greatly expanded, even MMA.
drug testing has taken a great toll on the carrib, usa, europe, that is until new drugs are adopted.
the "talent" now comes out of areas without a drug testing program with coaches that "know what they're doing".
you're really talking about small guys that could not make the pros in another sport mostly.
once in a while you have a natural that simply loves the sport, and finds team sports annoying.
these are the factors.