Dropbox bought Mailbox because, it wants to be more than a storage company and they are looking at diversifying their income streams with the rise of rivals encroaching their territory as well as, ensuring that people stay with Dropbox rather than go to other rivals. Acquisitions of Audiogalaxy, Cove, Snapjoy etc highlight that this is Dropbox’s plans going forward.
More often than not, Techcrunch states a disclosure of a Crunchfund investment within the posts that they are writing and although, they may have not followed through with this properly with Mailbox – you can clearly see on the Company Bio linking to Crunchbase that Crunchfund are/were an investor too[1].
Let’s not forget Mailbox is from Orchestra[2], a company which already had hundreds of thousands of users and is a company which won Apple’s Productivity App of 2011[3].
[1] http://www.crunchbase.com/company/orchestra
[3] http://blog.orchestra.com/orchestra-is-productivity-app-of-2...