Both teams have had the best development in the season that has just passed, certainly no one predicted that Bayer Leverkusen and Stuttgart would be teams that could compete well in the Bundesliga and get rid of Bayern Munich as defending champions because that is not an easy thing, often other teams just good at the start but slowly it will fail, that's why Bayern Munich continues to be champion, maybe Bayern Munich seems to be lulled because they are sure they will become champions again and don't notice that other teams have succeeded in improving.
Bayer Leverkusen may no doubt still be a good team competing here but Stuttgart, who knows whether they will still be good next season, especially if they are willing to sell Sehrou Guirassy because their rival clubs are really want to get the productive striker.
It is not only because of the development of Leverkusen and Stuttgart that they have been able to outperform Bayern Munich this season, but it is true that Bayern Munich's performance this season has declined slightly and they have not been consistent, making them unable to compete fiercely in defending the Bundesliga title.
Stuttgart is a team that has developed slowly over the past few seasons so it is very likely that next season they will also be better, especially as those who will play in the UCL will definitely build this team much stronger to be able to compete on the European stage.
Meanwhile, I am sure that Leverkusen will become stronger after Xabi Alonso decided to stay at Leverkusen, so Xabi's influence is very large, so if management supports Xabi in purchasing quality players, next season they will be much stronger.
In fact, I have doubts about Bayern Munich next season, they brought in a coach who doesn't have good experience, this is really a big gamble on what Bayern management will do even though they want to quickly recover next season.