Unpopular at Madrid because of really bad performances and gigantic wages. Other stuff like he spends all his time golfing and can't speak Spanish is like Lingard and social media, if the performances were good nobody would care.
He might have lost his hunger, 29 is hardly young for a footballer and he will be 30 next month. He's already moved to his dream club, won a huge amount, got one of the cushiest contracts in football... circumstances are certainly right for him to feel he's accomplished enough in his career. Impossible to say for sure though, I think injuries and age are bigger problems. He's been playing first team football since he was 17, 12 years, 550+ games and a lot of injuries later and he's lost a lot physically. His pace is gone, his stamina is gone, Bale used to terrify defenders with his running, how he's harmless. This is why he doesn't want to play as a winger anymore (which is a point the people who advocate us signing him for our RW often ignore).