Because of habit. There is always a cost in trying to learn something new, to the level you are on with some other language and it's tooling and libraries.

Programmers who program in some language for a few years realize they are still learning more about it, all the time. Therefore they know if they switched to another language it would also take them a few years to attain the same level of mastery.

And then there is similar inertia with an organization that has lots of code in some specific language already.