MPL2 requires that you make available the source of any changed files. Apple builds its version of LLVM off of a patched tree, and given that they're not particularly open about even the base revision of the public LLVM repositories their stuff is on, I can't imagine they would be happy having to publish those files publicly.
Yes.
We want folks to be able to keep their private chips private.
Past that, Mpl has some interesting baggage and requirements, and in the end, folks wanted something they all understood well, felt others would understand well, etc.
It's not a bad license, mind you. This was just the best path in terms of resistance, complexity, etc