Free, web-based email systems have more storage than you can use. They have powerful, accurate, swift search systems. They have clean interfaces, with threaded conversations and sane attachment management.
Then there's Microsoft's Outlook. Things have been getting better for those whose corporate upgrade strategy allows it, but with major updates happening every four years or so that's a long time to be looking at a non-threaded, licence-restricted storage- squeezed, treacle-slow-searching email system. Especially while the online services get better and better, and doubly so now that email is the single most important business application ever created.