Learning a new language takes time, which translates to money for companies hiring developers. Also, even many more senior developers prefer working in a single language due to the context switch otherwise involved.

Mature is a matter of definition. There are several apps in production already that are using Serverpod and are very happy with it. But, of course, it is a new framework and there are many improvements to come. It's always important to do your research before choosing any type of framework so you know it will support the features that you will need.