MMOs are a huge financial risk because they need constant upkeep in servers, customer service and game development.
They don't work without a playerbase, so there's the additional element of having to capture the zeitgeist otherwise the game is dead within two months.
Succeed and you have an addicted audience funneling you money long term, fail and you have an unsellable product.
I strongly belive that only large studios can make a successful MMO by piggybacking off a previously successful game or IP. That list is very small.
We are having a resurgence of single player RPGs right now, so why would Larian or CD Projekt Red or FromSoftware or Rockstar make a MMO when their next single player game will be hightly anticipated?
I think that the only plausible new entrant to the MMO market right now is Riot, look how cautious they are being, it's not guarenteed to ever happen.
The other best hope people is FF17 being an MMO in 5+ years time when FF14 is retired. The dev team showed they can develop a full game (FF16) simultaneously with a FF14 expansion and Square Enix have several other dev teams.
WoW I think is stuck in a nostalgia cycle now, the retail game is beyond fixing unless they can do a total old world revamp and go back to basics. I don't have much faith in that company to make a new MMO.