Can anyone explain the idea, why there is a need for another proprietary instant messaging platform, when there is open XMPP out there?

XMPP is made for interoperability. Proprieatry IMs are walled gardens, which is the dark age of computing. The same story was with e-mail in time of Compuserve and early AOL, when users simply couldn't even exchange e-mail messages between different providers. Luckily e-mail managed to break those walls and now no one even dreams that you can't send a message to user from a different e-mail provider.

XMPP (former Jabber) was intended to solve this issue for instant messaging. So it's really surprising that proprietary and non interoperable IM networks pop out here and there in this day and age.