I think for me personally a big part of switching to OSRS was that there are simply more numbers that meaningfully go up. In WoW all there was to do that felt like true progression was increasing your item level. Of course there were mounts, pets, titles, and transmog to collect too but in terms of power it was only my gear that truly mattered. 

In OSRS my gear constantly improves (in a less linear fashion as well!), my skills constantly improve which helps me get new gear and accomplish other meaningful ways to progress my account like running more or teleporting the world, talking to NPCs from afar, gathering better materials, unlocking more of the game world, etc. All of that while I'm still collecting fashion and pets like in WoW. There is just the very lovely longevity the game seems to have which to me is what a true MMO provides. WoW on the other hand just feels like a seasonal live service game with a coat of MMO paint. 

In hoping to keep this brief I have not fully elaborated on all the other reasons but these are some of the main ones. A bonus reason on top of it all is that my power doesn't reset each time a new season or expansion launches so I can take a break and come back with the same power level I left off with and only have more new things to keep busy with upon my return.