Google actively blocked a ton of their stuff from being accessed from windows phones. They even deprecated some communication protocols in gmail to ensure some features of Microsoft's mail app would not work with Gmail (and then Microsoft found a way to make it work with an alternative protocol and Google went ahead and dropped that too).

YouTube could only be used in internet Explorer. Google refused to let Microsoft make a client for it or do one themselves. Some folks created third party clients for it but Google was quick to block them too.

Then Pokémon Go came out for iOS and Android and it was the nail on the coffin.

Edit: I forgot to mention Instagram, it was a big part of it too. I think this was before Facebook acquired them, because Facebook (and Twitter) were VERY well integrated into Microsoft apps on top of having their own apps available.

In general windows phone was the easiest platform to make apps for (I made a few), but there was a lot of sabotage from those big names. I was very conflicted because in one hand, Microsoft was tasting a bit of their own venom - as they had done the same sort of stuff so many times before, but in the other hand windows phone really felt like the best mobile OS and I wanted it to stay relevant.