I thought it should be obvious. Apple was competing with Google/Android for mobile developers. For Android, you programmed in Java, which everyone learned in school. For iOS, this funny language with smalltalk syntax was a barrier to entry.
So Apple needed a nice language with C-like syntax to woo developers.
Plenty of Chris Lattner interviews answer that question.
I know Chris lather's intention. And I have read and listen to all the interview he did. I think the question should be, Why was it necessary for Apple to bet on Swift. Something I dont think Bertrand Serlet or Avie Tevanian would have done.
That is the answer right there, they weren't the ones calling the shots and the direction bought into Chris' intentions.
Oh well. I guess this is another thing I could blame it on Craig Federighi.
I asked this many times over the years, even in this thread [1]. Still no concrete answer. I still think overall it is a distraction.
[1] https://news.ycombinator.com/item?id=30418197