It looked to me like it was just due to recurring build issues. Lots of "swift can't import these conflicting C++ versioned libraries concurrently" and "can't use some operator due to versioning or build conflicts". Basically it sounds like trying to add swift to the project was breaking too many things, and they decided it wasn't worth it.

It's a shame, I think swift is an underappreciated language, however I understand their reasoning. I think if they tried to just use swift from the beginning it would have been too ambitious, and trying to add swift to a fragile, massive project was probably too complex.