Look at what we have now - basecamp, all kinds of free instant chats, group video chats, all kinds of organizational SaS, etc etc etc. Yet we still have ridiculously long email chains that often branch off and start to have a life of their own.
Why? Because
1) Everyone knows how to use email. Most employees don't want yet another way to communicate.
2) Your average office employee is pretty... average. And roughly half of the employees are dumber than that.
3) Training is expensive. One quick demo on a projector will not train your employees to use some new software. It's like half-life, 50% will learn after a month, 75% will learn after another month, etc.
4) You will always have employees who don't get this new software, many will probably not give a shit about learning a new tech, while they have a perfectly working way of sending a message. They will realize that when they change jobs, chances are, they won't be using this new software. But email will be there.
5) It is similar to the craigslist syndrome - it's awful, but everyone uses it, because... everyone uses it.