Why do they have no choice? Is just because they look better or is there more to it?
Because all the editing for TNG, DS9 and Voyager was done on videotape. Since the native resolution of videotape is so low, it - by definition - cannot be aired in HD. The resolution just isn't there.
Film, OTOH, has a much higher resolution even than HD. But if videotape is involved anywhere in the equation, that kills HD airing right there.
is the only ST series that was made from the ground up to be airable in HD. (Well, I suppose TOS could have been, as well, since it was done all on film, but some of its effects were so badly degraded that there would have been no point.)