Except when you realize all of those implementations needed to be coded by Microsoft. There was no way for a third party to plug in. I heard some things from MS people that the clients for IM services were driven server side which would have made it hard and inelegant to add additional protocols.

Nokia's maemo had this done with better execution. The SMS app had a plug-in for xmpp and I used it for Google talk. I think I used a third party one for Google voice. There was a Skype one that supported calling through the normal phone app but it didn't work very well. The clients were run on the phone and not in the cloud.

IIRC you had to do server side push as Windows Phone 7 didn't support local notifications and always-on Internet connection. Some IM clients used some tricks to run in background, such as masquerading as a streaming audio player (that had always-on capabilities enabled) but you lost the music player capabilities of your smartphone when running those apps.

WP8 relaxed some of those restrictions but it wasn't enough to truly develop a IM client.

It's true that only Microsoft could create such integrations, but it was a business decision. On Windows Phone 7 era, regular developers couldn't deploy native code and you couldn't call native APIs directly from the managed .NET/Silverlight runtime. Native SDK wasn't available at all, but it was a regular Windows CE at its core.

Maemo's was way superior to Windows Phone. It's a shame that Microsoft trojan-horsed Nokia.