Because having your entire business model depend on the pricing whims of another business (which in the grand scheme of things is a competitor, btw) is unsustainable. Getting out of AWS is exactly the right move for Dropbox's long term health.
There are many cloud hosting providers and they are competing intensely.
AWS marginal cost increase for supporting Dropbox is far below the cost of Dropbox building and maintaining their own solution.
Given these facts they should be able to negotiate a far better price/cost than their current solution of building it in-house.
There is nothing unique about their requirements which justify this move. It's a really weird decision. Probably we're missing something here. This must be part of some larger strategy they are executing.
As jamwt mentioned above, storage is simply the most critical element of their business, so it makes sense to control it as tightly as possible , similar to what Netflix does with their CDN.
The cloud is for stuff you don't care too much about.
