I'm sure there are reasons for it, but Chris Lattner has been jumping around a bit. Remember swift4TF.
But hopefully this one is seen through with lots of open source too.
but Mojo seems to keep all of that numpy cruft that's there because Python. (numeric computing was bolted onto python via numpy, but it's built-into Julia)
Edit: just saw this is a project by Chris Latner and Tim Davis. Hahaha ... that's immediate credibility.