A pure guess, but I would imagine fuel injection became universal on sprint cars and midgets due to the closer technical connections with Indy cars (engine-wise anyway) in the 50's? It would be fairly easy for a team to justify fuel injection on their Offy powered sprint car if the Offy powered Indy car had it, and sooner or later everyone is on the same bandwagon.

Carbs on late models have been part of the rules all along as far as I know, and would have made more sense in the 70's when late models looked like a street car with fat tires, so maintaining a stock appearing look with a carb would have been appropriate.