Where else is he going to go? There aren't that many top managerial jobs out there and even fewer of them are even potentially available. I suppose, PSG is a possibility, but Ancelotti is doing fine there and Jose may not be particularly excited about working in the Ligue 1 and living in France. He already worked in Italy and Serie A is not an attraction it used to be back in the day. Barca is a no go for obvious reasons and two Bundesliga giants don't look to change managers anytime soon.
Which leaves Premiership. Fergie stays put, City have ex-Barca people running the show, so no chance of that happening. I doubt he's interested in Arsenal and vice versa. Which leaves a certain club where he has a legendary status and is still loved by vast majority of supporters and personnel, so it'll feel like coming home. Abramovich, by all accounts, kept up with JM on friendly terms all these years and may be finally ready to start anew. Jose knows CFC has a quality squad and the owner is willing to spend to make it even better. It's a no brainer, really.