Err, because he believed last time when he went back that he'd get a chance at the first team....and ended up miserable in Sevilla instead. Whilst Emery is still at Barcelona doubt he'd trust their promises a second time.It's speculation also to believe the player will resist a move to get him back to Barcelona. If he's being told he'd be involved in the first team squad (or told they want him back to then sell on again to another CL club) then how easy would it be to decide to stay at a club not offering that that?
I'm speculating, sure. But there's more logic to it than believing a player would turn a move like that down and stay with Everton.
In Martinez he has found a manager who trusts him and brings out his best and if his 6 year relationship with his girlfriend is any indication he is a lad who values stability.