However, all post-production happened in standard definition, so all special effects, editing and colour correction would have to be redone, which is incredibly expensive.
They did that with TNG, but since those blurays didn't sell enough, they didn't bother doing the same for DS9/VOY.
This is a persistent myth, but false. DS9 and Voyager were shot on film, like TNG and TOS. TOS was also edited on film, TNG and later were scanned and edited on videotape. This makes it much more work to remaster TNG and later shows for HD, which was done for TNG but not later shows because the TNG sales plus the perception that the other shows wwre less popular and would sell less made it not worthwhile from a business perspective.