That WHY seems to only address a very small slice of the market. Some other WHYs I've encountered

- it was a way to outsource the mail server administration and already free in our corporate package

Which is pretty much all I have from the corporate side. On the personal or small business side its most often

- I don't really care that much about running my own stuff

- I needed a personal email address

All these are valid reasons/motivations but none explicitly argue that email messaging should become proprietary.