I don't think there is a single smoking gun- many things contributed to the downfall of Windows Phone. Microsoft had a decent market share with the old Windows CE based smart phones.

- Microsoft didn't care about mobile, thinking Windows CE was fine (they had ~42% marketshare in 2007)

- Windows Phone 7 was great, but it was too late by then (2010... 3 years after iPhone was release and 2 years after first Android phone)

- There were two resets (7 => 8, 8 => 10) which screwed customers hard. With the 7 => 8 upgrade, not only were apps incompatible, the OS was incompatible with previous hardware.

- The App Store was mostly full of garbage apps (lots of fake apps- hard to fine the genuine app)

- Carriers didn't do a very good job pushing Windows Phone (can you blame them? :P )

From my perspective (former WP user, 2011 - 2015), the biggest WTF to me was when Microsoft bought Nokia. That seems to be about the time they just completely gave up