> I'm surprised how few competitors there are in the Dropbox space.

There used to be many more - Sugarsync, AeroFS, Syncplicity, just to name a few - all bit the dust. Box.com found a niche serving business document flows; Gdrive, iCloud, OneDrive, all survived thanks to being features in a broader Big Tech suite. Everybody else? Outcompeted, plain and simple. Dropbox was just a cut above.

(I used to work at one of the companies named above, so although it's just one person's opinion, it's at least as informed as anyone else's here :) )