What it really comes down to is that these are three iconic races, and importantly with very long traditions. Look at the list of drivers that have two legs:
Tazio Nuvolari
Maurice Trintignant
A.J.Foyt
Bruce McLaren
Jochen Rindt
JPM
Of these guys only Foyt, and JPM have won the Indy 500.
How about the guys that aren't on the list:
Schumacher
Mansell
Prost
Senna
Stewart
Clark
Fangio
and more.
My point is the first list is not a list of better drivers than the one that followed. It is simply three races, that depending on your orientation you competed in the three, and maybe had a little luck (financial backing) along the way. Just for an illustration Maurice Trintignant competed in F1 from 1950-1964 he had two race wins, both at Monaco