Why did Microsoft buy Skype?

Microsoft bought Skype because Skype has 180 million people actively video calling. It doesn’t hurt that Skype basically pays for itself and is growing faster than Microsoft.

As cities move to 4G/5G and fiber:

• You want to offer people video chat at home, at school, and at work. Mobile phones, tablets, desktops, game consoles, augmented reality, televisions, everywhere. 
• You want to offer developers a platform for building video chat into other apps. Scriptable clients, browser extensions, hardware embeds, and cloud services; all on Skype’s roadmap.  Skype can’t offer contexts to trigger and shape conversation, but a million Microsoft developers will.
• You want to offer Microsoft’s product managers new conversation features. "Better with video chat" can be tried across Microsoft’s hundreds of products for the next three years; we’ll walk through some of the likely in-Microsoft partnerships in another post.
• You want to offer subscriptions for premium services that pay for basic services. Freemium bootstrapped Skype and should continue as Skype adds more premium services.
• You want to be making enough money that you can pay off greedy carriers (assuming failure of net neutrality) and governments that choose to tax.