When he became the only game in town.
Top club needs a striker. We're the only top club in that position. He and Belotti seem to be the only ones available (I don't believe Aubameyang was ever an option).
Personally, from what I've seen, Belotti would seem to fit the system a bit better (work, work, work), but who know's what really going on there?
I think he and Michy give us some flexibility in the squad. Morata, Hazard & Pedro on the counter will be deadly.
I have to say, I'm not overly excited, especially at that price. But, better than a trolling Costa.