Also, TOS exists entirely on film. TNG exists 99.9% on film. DS9, Voyager, and B5 have a large number of film elements, but a great deal of the FX were CG and only exist as broadcast video tape masters. So you would have to recreate the CG shots from scratch in order to convert them to HD. The film just has to be rescanned, like with TNG. But it is the expense and time of the CG recreation that prevents it from happening. We got lucky with B5 that JMS convinced WB to do high quality conversion of the original master video tapes (making it as high quality as any shot on tape series of the day) that resulted in being able to see how fantastic it looked originally rather than the horrible uprez job done for the DVDs. We can hope that DS9 and Voyager get at least that quality, which is suitable for streaming.