Not so much a thing I want to know, because I already know - but it's potentially a question that could come up later, and I found it quite interesting when I read about it the other day.
Anyway, apparently, there are a lot of old TV shows and series that will never appear on Blu-Ray or in HD or in UHD, but on the other side of the coin, there are many old movies that have been converted to HD or 4K and the quality is superb, and some of the old classics are also still to be converted, so why can't everything be done?
Well, the answer is pretty simple - film. If something has been recorded on to film, it can actually be blown up to around 8K without a noticeable loss of quality, because of the high quality film and the size of the grain when they were first made enables it, and it's very easy to digitise to HD/4K/8K.
But, because of a lot of TV shows that were popular from 50's onward were recorded in to tape (with the exception of outdoor shots which were easier to shoot on film), they are already electronically recorded in 480p, the electronic signal cannot be converted to anything higher as there will be a huge loss in quality. The resolution is permanent.
So there you go - that's why some of the Star Trek series and such like will never appear on Blu-Ray and other HD formats.