Simply put, Gmail provides a compelling user experience:
* fast and accurate search. This is key, and I don't know of a single desktop email client that even comes close to providing a decent search feature. Decent here means "answer full-text queries accurately within a second or so". I didn't realize how useful this was until switching to Gmail; now I have it, I can't switch back. I don't even bother organizing my email more than a minimal amount any more, I just search for things when I need them.
* remote access is also key. I regularly access the internet from at least 4 different devices, on at least 3 different operating systems. Sometimes I do so while traveling. Gmail makes this convenient --- all I need is a web browser. IMAP need not apply --- I found IMAP appallingly slow last time I tried it.