Probably for the best. They realized, "Oh we can't just shit these out and would actually have to work on them? Nevermind then."Apparently, GTA Definitive Edition has made Rockstar rethink a few things.
Red Dead Redemption, GTA IV Remasters Shelved After GTA Trilogy Debacle
Sources tell Kotaku that Rockstar Games is focused on GTA 6 and not the past, for nowkotaku.com
So because it would take effort they are not going to bother because they need a new GTA online.