Not every player is fit for every team but Bale situation wasn't being able to fit in there or not. Zidane played him very well at one point and dude won them a champions league just a few years ago, however when you look at what is going on at the season afterwards you will see that he wasn't really liking playing there, which means that there could be something that was about team and player having a problem between each other and Bale wasn't playing his best ball.

After he moved to Spurs he was once again very happy to play and that showed on the pitch as well which is what I believe is the main reason of him getting better once again. He didn't become a bad player over just one off-season after the champions league win, he was still good, he just didn't want to play and when he wanted again he showed his true colors and that is why we see him better.