1. Stickiness.
Google Docs (and even Microsoft Word) work so well when you use them in conjunction with their respective ecosystem. Many users choose Google Docs over Dropbox Paper simply because they already have Google Apps accounts, even though as a product what they really want is something more like Dropbox Paper.
Inevitably, what happens is that those who can try out a new product on a small team do so, but those who are forced to work between teams or even between companies choose the solution they're already using; i.e., the stickiest.
2. Plain text is powerful
I'm sure I'm preaching to the choir here, but each new "office productivity" or "word processor replacement" tool comes with it's own silo. Even Microsoft Word, which has it's own file type that you can save to your Desktop, is largely a proprietary format. You can export data out of Google Docs or Dropbox Paper, etc., but the idea is that you're leaving the promised land. You're packing up and moving somewhere else.
On the other hand, we know from the Unix philosophy that plain text is a universal interface. Using plain text can help us escape these siloes. The problem, of course, is that from grade school we teach students that to edit text we use Microsoft Word.
There are a lot of cool new products in this design space of "word processor replacements," but few which tackle these two fundamental issues.