Right, let me tell you why Carlos Ancelotti has been failed at Real Madrid this season, although he has much better management career than Xabi Alonso and also has a better squad depth than Xabi Alonso. But he failed to manage the team well....it is because sometimes old tactics didn't work well, although you have a good team. So, the same thing happens with Angelotti. He tries to get Real Madrid to work on the same old tactics over and over again. But in fact, one tactic doesn't work over and over again, no matter how legendary a coach you are.
So it means to say that sometimes even great coaches fail. Because he cannot correct his mistake and makes repeated efforts on it. The same ideology does not save you on every perspective it is necessary to change, otherwise, like Angelotti, he can also fail. Bayern Leverkusen plays in the Bundesliga, which is usually not that tough, while La Liga is a tough league. You have to face many competitors, and a small mistake can defeat you.
Since you know why Carlo Ancelotti failed. I want to also know why Xabi Alonso failed to repeat his outstanding performance last season. Was it also because of old tactics? So many factors can affect the performance of a club even when the tactics are good. And from my observations, injuries played a great role in the poor state of Real Madrid this season. Real had to struggle with injuries that affected defenders like Daniel Carvajal, David Alaba, Éder Militão, and others. And the mistake the Italian coach made was his reluctance to get new players when the transfer window opened briefly in January.
I don't know the parameters you are using to grade these European Leagues that you are saying that the Bundesliga is not tough. The German League also has some strong sides that can compete with other top European clubs. Ancelotti might have made some blunders but I don't think he is a failure because he failed to win a trophy in one season.