DropBox nailed it in replicating all the Windows’s share sync features, but doing it over HTTP to a web server with a web interface without the need to have someone from IT set it up. They nailed it again by spreading to multiple platforms, particularly the mobile ones, which lets people access and use their files from anywhere. And then nailed it once more by introducing file version history.

At this point, yes, Windows has the same, or arguably more, features… but where those features really shine, is on mobile, and DropBox is king there, so it’s only natural that people would use it everywhere.