the company started back in 2012 and raised most of that money then, the market probably looked more attractive at that point to VC's (Dropbox valuation was shooting up) but they took awhile to release anything as it's now 2016. While at the same time, the market outlook has soured in general for file storage companies is the narrative at the moment, Dropbox has now shifted most of it's focus it seems to it's enterprise offerings as the consumer side isn't a sustainable revenue model to get to an IPO.