Just a thought... But...
I'm sure both Fox and Disney would love to sell the original versions of the trilogy since they only care about making money and selling the original trilogy would be a cash cow. However, I'm willing to bet that Lucas foresaw that possibility and he likely included a stipulation that says Disney and/or Fox cannot release the original trilogy without his permission otherwise the rights to the films revert back to him.
He may not be a good filmmaker, but he is a savvy businessman.