One big difference between the PS1 classic and the NES/SNES classics is that there is actually a lot of pent up demand for these old school 2D games. The game industry rarely puts a serious effort into these types of games anymore and when they do, they often get a lot of unexpected success like they did this year with Octopath Traveler. There is an underserved market of people out there that want 2D games to be flagship games again instead of just letting indies make these games and then it's digital only versions.
PS1/N64 era was the beginning of 3D gaming. Those types of games are really common now, and often a lot better than their PS1 counterparts (e.g. Tomb Raider). There isn't really pent up demand for these types of games right now.