Remember when AJAX was supposed to make pages faster? LOL. Of course, back then we were just sending HTML to be injected, not JSON that has to be processed by 50+ Javascript functions before finally becoming HTML (or, some DOM entities, anyway).
And offers far less features. Attaching a file to an email takes ages without drag and drop, this alone is worth waiting few more milliseconds to most people (Gmail loads in less than 2 seconds on my crappy hotel wifi)
Most of the time you read e-mails, not write them.
When you write e-mails, most of the time you don't need multiple attachments. The first attachment is reasonably fast to add.