The problem was actually very specific to that area. Before the nineties, pretty much everything was shot in film, which can be converted to HD quality (assuming 35mm or better film).
In the nineties, CGI started to become commonplace. However it was still very computationally expensive, as in minutes per frame rendered. So they rendered everything only SD format. Most action scenes were still shot in film, but when editing they would overlay the CGI and again render the final output to SD only. So during the (relatively) short time window where CGI was new and HD wasn't a thing yet, all content produced can never realistically be made available in HD. While all content before/after can.
(You could still redo the editing process of course and render CGI in HD, but that is such a crazy amount of work assuming the source material even still exists, that it will likely never be done but for the most desirable content)