Because he is by far, the worst performer from their senior attacking players. Ronaldo, Isco, Rodriguez and Benzema all play better than him.
If people can only understand this instead of claiming that Madrid fans are idiots, it would be quite easy. But again, in order to understand this you should watch Madrid play instead of claiming how great Bale is based on his last season for Spurs and on him being British.
I watched a fair bit of Real Madrid last year, and while he underperfomed - he wasn't vastly worse than other players mentioned. And he was still better than any of our attacking players barring a couple.
And even if that was the case, I go back to my previous point - one bad season does not make you a bad player.
Di Maria was, I think its fair to say, a failure at United last year. Barring a few half decent early performances he looked uniterested and struggled with the pace and strength of the Premier League. Yet PSG are still willing to pay a large transfer fee for him because while form is temporary, class is permanent. Di Maria has clearly struggled to fit in and had family issues which affected his game. Who says Bale isnt in the same position?
The fact is, his poor season doesn't take away the skills that he has, nor his ability over all. Infact, if anything you could suggest that time in the pressure cooker of Madrid, along with playing alongside much better players would improve his game long term.
There are some bizarre arguments here. Some seem to think that one good season from him doesnt make him a great player, but apparently one bad season means he's finished? I don't understand that at all.
He's a player who's strengths make him not only cut out for the Prenier League, but also mean he can add the direct pace and ability from wide positions that we lack. And just because he's not currently lighting up La Liga, does that mean he won't come back and light up the Premier League? Not for me. If he's back in a League where he's comfortable, has the support of the fans and under a good manager who can get the best out of him then I'd suggest the chances are he'll do very well given the skill set and ability he clearly has.