I think the real problem is email. It's this antiquated technology that doesn't fit into the modern software as a service tools everyone uses to get things done. Email worked in the world of desktop apps but the snail mail paradigm email emulates is antithetical to the real-time fast paced team collaboration everyone is trying to achieve.

Yet nothing manages to replace email, we depend on email above all else and what happens is that important information about tasks leaks into email from project management software and there's no easy way for this kind of software to fold email into what they're trying to do. Email doesn't play nice with anyone, it doesn't even play nice with email servers and email clients. Yet we all use it. I doubt anything will replace email. 200 years from now space ships on their way to Vega will be sending IMAP mail messages to planet Earth.