Basically people aren't going to switch to a new way of doing text unless someone forces them. Only a very small minority of people use something like FreeMind or EverNote instead of Word as their primary writing tool, so how could appjet ever hope to get more than a tiny fraction of those people when their best functionality only works when you have multiple people?
The only way to get mass adoption would be to force people to use it, which is possible, but only if you design the company around that from the ground up. Which I don't think they were organized to do, at least as far as I can tell.
If Google wants to, they actually have the ability to force more people to use it. Not sure if that will happen, but I hope it does since I wish I were able to use it more myself.