Why Fox/Disney haven't:
1) Colourists aren't cheap
2) They'd need to remaster the Blu-Rays with the new file, which would again take time and money
3) They'd then need to spend money advertising the fact they've remastered the film to try to get people to buy it
4) It wasn't particularly popular so there's not enough market to justify 1-3
Why nobody else has:
A compressed video file does not have the same depth of colour information as the master files. If you tried to brighten the footage as-is you'd mostly end up with a very grainy mess. Here is me pushing the image without destroying it. Better? Sure. Good? Still no.
Also, time. If someone sent me an HD blu ray rip of the film I could totally go and try to push the dark parts. But I also have better things to do with my life.
(Unless someone wanted to pay me, then sure!)
I think you should do it, consider it a hobby
Show them filmmakers how a film should look. We can call it the SIL cut then