Same reasons MS ended up buying Skype, despite having a near decade lead in tech (NetMeeting before MSN Messenger), 300M active users, etc. etc.? MS even had VoIP-to-PSTN in the 90s, albeit via a poor partnership with MCI.

Same reasons MS ended up investing in FB, instead of turning Messenger (think: you've already got everyone and their friends using it!) into a good social network?