I don't know how the split happened in first place, but what always intrigued me was that after the split IRL was for American-born drivers only, while Cart was all foreigners, plus 3 or 4 Americans. Was it only the total lack of interest on the all-oval racing series, or were there other issues.
In a similar context, I saw some dumb comments from Nascar people on TV (no clue who they were), where they were totally against a foreign manufacturer. How do you prove you're the best if you refuse to compete?