Because most of the world has gone to webmail, especially Gmail. You don't need an e-mail client then. Fewer programs to maintain, fewer potential exploit avenues and/or points of failure.
I reckon there are still a lot of people who haven't/won't go down the Gmail route. A significant number of people would rather have the overhead of maintaining and securing an email client, as opposed to letting a 3rd party scan all their email and run roughshod over their privacy.