So, you have users who have years of e-mail in GMail, use GMail for free, use their @gmail.com everywhere, and probably don't know how to migrate their e-mail to another provider.

Give them a choice: (1) go through a messy migration or (2) install another app. Most will probably do (2) without even thinking about it.

And then you have to factor in that worldwide, over 80% of the smartphone users already have a platform where GMail is the default mail app:

http://www.idc.com/getdoc.jsp?containerId=prUS24442013