My thoughts   

Opperman4x pretty much sums it up. Dirt miles were removed from USAC championship, CART was formed, increase in number of road races, increase in arrival of funded foreign drivers.  American drivers with talent, success in sprint cars, midgets no longer were pursued by Indycar owners. The Indy Racing League in the late 90s gave some opportunities to American sprint and midget car drivers but that evaporated soon after upon merger of IRL and CART. 

In this year’s 500, only Ed Carpenter started out in sprint cars. Being the stepson of Tony Hulman George provided him with the dollars required to pursue and further his career into Indycars. Today he is a successful Indycar owner, driving only at the 500. He has won the pole at Indy three times and is a top runner each year at the 500.