I'm not 100% sure but I thought Mailbox needed access to certain things that are outside IMAP to do what it does. For instance, it makes its own folder in Gmail that it interacts with, and I'm not 100% sure if "Archive" is a standard IMAP function.

I'm not an IMAP dev so I'm open to hearing differently from someone who knows better, but it was my understanding a network-neutral IMAP solution wasn't tenable.