The death knell was Elop announcing that Nokia would be exclusively Windows, at a time when Windows had low single-digit market share and where this exclusivity was a one-way street. Other major manufacturers continued to ship Windows phones, and they did that in parallel to their main Android line.

I never understood what the end game here was. If they had wanted market dominance based on someone else's OS, they could have just used the well-established Android. There was little to gain (their efforts of promoting Windows Phone would have also helped the competition's offerings) and all to lose.