I think ronaldo wants this move for two main reasons.
1) he is determined to win a 6th champions league to equal the record of most champion league titles a player has won and to stay ahead of MessI.
2) To win the balon dor once again.
He will know the best chance he has of achieving both these are to join a club like city.
Only obstacle we have left imo is if emotion gets in the way and United offer a crazy bid and we walk away. But if ronaldo is the type of person I think he is, the opportunity to win one more champions league will be the deciding factor in joining us.