I know...20 years ( atleast) is a huge gap between triple crown winners.
Seems to run in cycles. There were 4 in the 1940's, then a 25 year gap from Citation in 1948 to Secretariat in 1973. There were 2 more in the 70's (Seattle Slew and Affirmed) B4 the current drought, which will be @ least 30 years come next spring).
Personally, I wish they wouldn't breed thoroughbreds, who have those fragile lower leg bones that lead to so many suffering fatal injuries. (Most of whom are euthanized in the medical van right on the track). The standardbred horses seen in harness racing would do just fine under a jockey, and are nearly as fast.