I think the lack of meaningful updates had an impact and then certainly R*'s announcement of stopping all new developmental updates of the game.
As someone who had started laying since RDR2's release and then RDO when it was made available, it started getting old after a while. There was a lot of potential but simply not the extent of support GTAV has had which in fact I still get back into from time to time.
Still an epic game and worth every penny IMO. I'd expect an announcement of RDR3 would stir things up though. ....or maybe R*'s rethinking of new updates for RDR2/RDO.