Kilroy raised an interesting question, so I went for a look. I took the top 15 riders in the championship top class only in 10 year increments, starting in 1980. It seems you are correct, at least about the last 20 - 40 years even. My guess is that we saw a few things happen.
- The USA for a long time followed their own championship. Own classes, etc. It also seems that they had, for a period no or very little feeder classes.
- The UK went toward WSBK in terms of racing classes. See how they rule the roost in WSBK.
- Spain started a racing academy and a Spanish championship almost identical to the MotoGP championship smaller classes.
- Italy recently gained the VR46 Academy for Italian riders, but they also have a championship similar to MotoGP smaller classes.
- It is much cheaper and easier for an Italian or Spaniard to get into and participate in the various " beginner" classes such as Red Bull Rookies Cup, Junior World Championship, Spanish or Italian Moto3 Championship.
1980:
Champion = USA
US = 2
GB = 1
Italy = 5 (interesting this includes one Graziano Rossi who finished 5th)
Netherlands = 3
Venezuela = 1
NZ = 1
Switzerland = 1
RSA = 1
1990:
Champion = USA
USA = 4
GB = 2
Italy = 2
Aus = 2
Spain = 2
France = 2
Brazil = 1
2000:
Champion = USA
USA = 1
GB = 1
Italy = 3
Spain = 2
Brazil = 1
Aus = 1
Japan = 4
France = 1
Netherlands = 1
2010:
Champion = Spain
USA = 3
Italy = 4
Spain = 5
Aus = 1
France = 1
Japan = 1
2020:
Champion = Spain
Italy = 4
Spain = 5
Aus = 1
France = 2
Portugal = 1
Japan = 1
RSA = 1