Seems like an educated guess at this point, though I’d love to be wrong.
There’s nothing educated about the guess, though. It’s literally a reaction rooted in personal headcanon/fiction and nothing more. Someone asked themselves on a forum once “what could possibly prevent me from getting the product I want to purchase” and the first thing they came up with was “Lucas put a clause in the contract.” It sounded good enough, and a lot of people ran with it. But it’s not sourced to anything. It’s literally spec that people decided to accept and promote because the notion that Disney just doesn’t want to contradict Lucas’ wishes was too unbelievable for them.