I personally use Dropbox, mainly because it's easy to have multiple people logged in under 1 account without interfering with anything else. For instance, I have both my wife and my moms cell phones set to upload pics automatically to my dropbox account, so we have 1 central location for family photos.
I couldn't really do that with Google Drive, because on our Android phones, google drive access is tied into your google account that you are logged in as on your phone, so in order for them to log into my google drive, they would also then have access to everything else tied into my google account (gmail, google music, google books. google+, etc). Sometimes having SO many services and programs tied into 1 account isn't a great thing, if you want to share that account with other people for 1 specific purpose(like I do with dropbox).
For people that aren't tech savvy like my wife and mom, there's no way I wanted them constantly logging out of their own google accounts, and into mine, just for the sake of pictures. I'd be doing tech support for them constantly when they couldn't figure out why they lost access to their gmail or whatever.