This seems to have been due to the Gygaxian Ego Problem. He had a high opinion of himself as The Creator, and was quite unwilling to adopt changes that hadn’t come from within his creation. So other people’s ideas that had been published in Dragon, no trouble. Separate games, a serious problem.
There was a quote from him in the early 1990s. He was at a convention, drumming up support for his Dangerous Journeys RPG, which hadn’t yet been published. Extolling the virtues of the new flexible magic system, he was asked if it had similar capabilities to the Champions power design system, which is very capable. His response was to the effect that while he wasn’t familiar with Champions, he understood that it used character stats (Strength, Intelligence, etc) in the same basic way as D&D, and therefore he wasn’t basing stuff on it, they were basing stuff on his work!
It appears the session ended quite swiftly after that, as people realised they weren’t going to get much in the way of answers.