One reason is likely their experience with IE. They took so much flak destroying Netscape that it's just cheaper and less stress to buy the market leader and get on with it.
In terms of 'why', this is just the easiest way to integrate telephony with their own operating system in a way that leverages the network effect. Soon, any app which runs on Windows could just use the Skype Service. Mac/iPhone/Linux will only have bolt-on app integration with the market leader, or have to develop their own from scratch. And Microsoft will slap some very serious marketing behind this. Maybe Windows phones automatically use Skype when you call other Skype users and they're online, saving costs. Or MS could integrate it into business services, taking over internal phone networks. Maybe they have thought of a use I haven't in my 5 minutes of thinking :)
In terms of 'why', this is just the easiest way to integrate telephony with their own operating system in a way that leverages the network effect. Soon, any app which runs on Windows could just use the Skype Service. Mac/iPhone/Linux will only have bolt-on app integration with the market leader, or have to develop their own from scratch. And Microsoft will slap some very serious marketing behind this. Maybe Windows phones automatically use Skype when you call other Skype users and they're online, saving costs. Or MS could integrate it into business services, taking over internal phone networks. Maybe they have thought of a use I haven't in my 5 minutes of thinking :)