On top of that, Google Drive makes sense to Google for their multi-devices/Google Access Everywhere kind of strategy. When you have tablets or phones with limited memory but network access, Google Drive plays well in that space to extend the capabilities of the said devices.

Dropbox, on the contrary, only has a SINGLE service to rely on, and they could be made obsolete by pricing, features, integration or other stuff like that. It's not good to rely on a single service for your bottom line, and Dropbox is no longer a small startup just beginning on the market.