alt Hacker NewsWindows Phone was dead in the water because many services did not have first party support, and the third party clients kept getting killed / people banned from said services.
Google was extremely aggressive in muscling Microsoft out. They refused to release a Gmail, YouTube or Maps client for Windows Phone but made sure those services did not work (properly).
And indeed on top of that, Microsoft switched UI frameworks 3 or 4 times. And they left phones behind on the old OS releases repeatedly, that then couldn't run the new frameworks.
Still, Windows Phone its UI concept was really great, and I sorely miss the durability of polycarbonate bodies versus the glass, metal and standard plastic bodies of today.
What burned me was that there was no updating from WP7 to WP8 - After playing around with one and genuinely enjoying the experience, I convinced myself to buy a Lumia 900 in April of 2012, just for Nokia/Microsoft to effectively say "that was stupid, wasn't it?" when the Lumia 920 and WP8 launched just 7 months later. Releasing a so-called flagship device that they knew would be incompatible with their upcoming OS, effectively killing software support before the year was even finished, really doesn't inspire confidence in the longevity of a product.