The thing with Mailbox is that it was truly a great product before it switched hands to Dropbox. Once they bought it, that was the end of good functions and the product only went downhill from there.
There's a lot of potential with email clients that will help you work better, Mailbox was definitely one of those products that helped. With good clients for Mac and PC along with smartphones it would also be profitable IMHO.
The only thing I can think of is that Dropbox is headed for major firing rounds and they want to save up on resources.
I have switched to Airmail on my Mac and I am much happier now.