Because it's just not that much money in the grand scheme of the deal. The number of people who give a damn about the original films is a tiny, tiny percentage of the Star Wars market. Regardless, we'll never know. All that I'm fairly sure of is that we'll never see those original versions...
I doubt it will ever happen as long as George Lucas is alive. It wouldn't surprise me one little bit if he and Disney have a handshake deal to not release the OCs while he' still around.