Because he plays like shit. He has speed but it isnt useful in spain because technically bale is a worse footballer and in spain there are too many technicall players.
The Problem is that English league is an anti-football league. Its about freaking physic so any decent technically team will outpass them to death. Even Kiew and Basel fking owned everton and liverpool
He wont move , wolfsburg wont sell him this year. Wolfsburg have so much money, money is irrelevant for them. And if wolfsburg get a good striker, he has more chance winning trophies and Champions league then city for example.