1. Market reach, it's where the customers are. Compare the sheer number of apps that have native Dropbox integration, with those that have OneDrive. While it's far from scientific, nearly everyone I know has a Dropbox account - very few have a OneDrive account.

2. It makes their own OneDrive service seem even more competitive. Why pay for Dropbox when you can buy Office365 for about the same price and get unlimited storage?