The reason is open protocols soon become the single biggest blocker to innovation. Open protocols stagnate and ossify. So some competitor with a proprietary system will come along and eat your lunch. That's why there have only been 2 changes in email clients ever - the move from terminal to desktop GUI and desktop GUI to web 2.0 client. That's why XMPP and IRC clients will never compete with slack (queue the 'what does slack have that IRC doesn't have thread').
I don't think it's as much that the protocol stagnates as much as it is that companies have a strong incentive to get users into their walled garden. If you use an open protocol, you can't lock people in.
We'll be trying to fix that by offering the XMPP "Team Chat" client that is much needed... https://nayego.net/ you can subscribe if you want to see the preview.