When you really think about it, email sucks as a collaboration tool. Either you need to keep copying everyone on every message so they know what's going on, even if you're just passing on a message, because some people might not know that someone else has been informed of something. Maintaining that list of who needs to know what (and who needs to know who knows what) is a major hassle.
That's even before you get to the point of editing documents as a group, where multiple people can be emailing multiple versions of multiple documents so often that it's not even clear what the most recent versions are, let alone whose job it is to merge them into one.
There are solutions, but they tend to be online (where businesses don't trust the data storage) or don't mesh with Microsoft Office. So we stick with that one old tool, email, that doesn't quite get the job done, but manages to do most of what we need.
Mokalus of Borg
PS - The core problem is solved.
PPS - It's the migration and market share that we need now.