My guess would be that George Lucas was an actual human who was embarrassed by the Holiday Special, and so as long as he controlled the rights, he kept the Holiday Special out of the public eye.

Disney, on the other hand, is a corporation that will happily sell anything to the public if the public will buy it.

They have even said essentially that. They cancelled the rest of the 3D conversions in order to focus on the new films.

EDIT: totally repeated what you said. i can haz reeding compreehenshun

Gee, someone should do a kickstarter to raise money to pay for the restorations, so Disney can just release them

As I recall, Lucas' answered the question of "you scanned the masters to make the special edition, so why not just release that scan as a restored original?" by saying "those scans were irrevocably turned into the special edition as they were restored, so there's no restored original." Some people interpreted that to mean the master film elements were destroyed.

However, that is immaterial. When Disney is ready to make more money off the effort, they'll have someone rescan the masters at super-high-res (16k maybe) whereas Lucas had scanned them at, I think, 4k. (Remember the timeframe we're talking about here.) Then they (Disney) will control the nature and extent of the restoration, something they are VERY EXPERIENCED AT.