Because it's already there and it already works just fine.

It's so frustrating to see this sort of comment on nearly every post about a language outside the mainstream. It's a religious argument and not a productive one.

Well clearly it didn't work just fine if its threading model was interacting poorly with that of their Haskell code. If there's a mismatch between those two components then that will continue to bite them in the future, and it's legitimate to ask whether moving the boundary would be a better way to solve the problem.

When they realized the bug the path forward probably seemed like patching and not a Haskell re-write, and that's a reasonable mistake to make.

They seemed to have succeeded in creating a culture of coders/hackers that were opinionated/empowered sufficiently to use their tool of choice, and to persuade others of the advantages.

This doesn't really happen in large-cap companies (though does happen to empowered teams, but even then, only somewhat - thinking finance). And speaks to advantages.