I've been addicted to IMAP since my university days with Pine. Apple Mail had IMAP support when I first came to OS X, while very few other clients did. I've stuck with it, especially since it comes with the OS. It also supports multiple mail boxes which I relied on early too.
And I like the native UI, how it responds to drag and drop, interaction with the Dock, works with built in OS services, etc.
Also, relatively fast mail searching via Spotlight kept me hooked on mail. I suspect everybody else has caught up now.
It's not without its own set of faults, but I find it nicer than Mail.
The lack of decent Exchange integration does let it down, but that's fine.