Maintainability. If you're the only person who can edit the library, you've just created a bus factor that would be unacceptable to most project managers.
This is my personal opinion, but I think that domain expertise is a much stronger component of bus factor than language choice is. You can write unmaintainable code in any language. But good code often still has prerequisites to understanding. Meanwhile, if you have the domain expertise, you can get quite far in understanding semi-bad code just by pattern matching on manifestations of your domain.
And I say this as someone whose full time job was to do research prototypes of robotics libraries for a while.