If it's more than 10-20% of Dropbox users, that says pretty bad things about their market share.
Based on what?
Safety.
Edit: It was probably a reference to that it is "preinstalled" on retail machines.
Given Apples 7.6% share of the 'PC' market as reported in 3Q15 (http://www.gartner.com/newsroom/id/3146617) I wouldn't be suprised that the vast majority of dropbox users are on Windows, simply because the vast majority of computer users are.