because it lags like hell (search is almost unusable due to lag), it takes the system down with it if you have thousands of mails, you can't edit quoted text (or even copy it for interleaved posting), you can't add attachments from the filesystem (only 'supported' files, try adding 2 tarballs as attachment to one mail, impossible)

The only good thing about it it is that it's superfast in notifications. It even beats gmail web app.

Edit: and because it confuses the "scroll this mail horizontally" and "go to previous/next mail functions"