MXNet existed before AWS picked it, and it has a lot of strengths. I’d use it (especially with Gluon) over TF any day. But that said, PyTorch is usually easy to use on AWS... the preference for MXNet seems weak
It existed at CMU, but it seems like even CMU has moved over to PyTorch. I think Amazon just doesn't want to seem like an "also ran" by conceding to one of its competitor's frameworks.