For me, there's three reasons:
1) they seem much more accessible to people with @outlook.com, @comcast.net, @yahoo.com, etc. email addresses
2) On a Mac, I've found their syncing to be quicker, more reliable and more efficient than Google Drive. Google Drive does not like network glitches at all, often having to be killed and restarted. I've tried changing ownership of things in Google Drive. Sometimes I can, sometimes I can't. This is a problem when working with things I want to hand over to someone else (for me that's the soccer club, PTA, etc.)
3) I had separate apps for Google Drive and Google Photo backup. Which I liked. Now I have Backup and Sync or whatever they're calling it. Dropbox has been easy to set up and forget.
I have more stored in Google Drive because I buy Chromebooks for me or the kids every couple of years.