I don't know about the regulatory guidelines which would or would not have barred Elon from exercising any sort of control in the board, but my sources from Twitter are telling me that folks were not happy about the prospect of Elon wanting to change Twitter to be in his image and it's likely this that the higher-ups communicated to Musk and told him to please back away (hence the 'I believe this is for the best' affirmation line from Agrawal himself).