Yes those resets were bad, but another factor was arrogance. They thought just because they're the ones with the dominant desktop OS, they'll automatically win the smartphone market somehow.
So if we ignore that stupid WP7 thing, they were 4 years late to the party. There were two players already in the market, the dominant one open sourcing the core of their OS, giving OEMs a lot of freedom for customization, users can sideload apps easily, the SDK was free and you could start testing on your device right away or distribute the apk directly.
Microsoft then thought it was smart to require a registration (for $$$ iirc) just for starting to use the IDE. You could only push one app to your phone, tied to your dev account, at a time. Sideloading didn't exist. That probably helped adoption a lot /s. Device pricing also didn't seem too competitive. At least with Nokia they could have done much more agressive pricing to gain market share. But the Nokia aquisition was the next stupid move: Nokia now got Windows for free while others still had to pay licensing fees, pulling a 3dfx Voodoo 3. Surprise, most switched to Android exclusively.
Instead of trying everything to get users and devs on board they copied mostly Apple's policies, who could really only get away with it since they were there first to market, and already had an established user base when Google came along and gave Android away almost for free and made copying (pirating) apps as easy as possible. Microsoft pretended they already own the place from day one.