Surely something must have motivated him to go to Real Betis. Sometimes there are factors we don't know about in the decisions of players and coaches. For example, Mourinho refused to go, despite being overpaid, to play in the Arab League. Maybe his wife doesn't like the climate or something else in that part of the world.
Yeah, something must have made him to go there, like you said different factors plays a key role in moves like this. Last season when he was struggling at Manchester United, Real Betis came in and offered a deal for him, he came into the club and impress and the club showed interest in wanting to buy him. It's just natural that he honours it and again, maybe another bigger club didn't come for his services that's another factor too.
For me I will say that Real Betis were the club that gave him a chance when he need it the most, because at that time things were not going well at Manchester United for him. This kind of trust means a lot for a player who wants to regain his confidence. Apart from that, when he was in Real Betis he gets regular playing time, something that might not happen if he went to a bigger team.because for him Playing every week can be more important than just sitting on the bench at a top club.
if a bigger club had come in with an offer, he might have gone there instead. But since that did not happen, it made sense more for him to stay where he was wanted and where the coach believed in him, than were they won’t value or even believe in him.