Except if, uhhh, they have made too many assumption on Gmail features being available, so that from the backend to the UI layer, it needs a good, over-through, rewrite to work with anything more standard.
Gmail has spent a lot of time making it work well with web mail clients, be fast etc. Other people have not done this much in general, they would probably have to buy someone.
But Mailbox only works with GMail email accounts. I wonder how many users Mailbox would have if Google shut them off.