From what I could understand, and from his own words in his older interviews - he's sick of being Bond. He wanted this to be done long time ago, but seems like contract with a studio got in a way. Or something of sort.
From the man who said "... I'd rather slash my wrists than play James Bond again"
"If that wasn’t enough, the 47-year-old admitted that if he did return as Bond it would be just for the money. He took a similarly dismissive opinion on that staple of British conversational life, who should take over in the role: “Look, I don’t give a f-word.”