Having worked on the Drive team, I can assure you Google Apps (which includes GMail, the Docs editors, and Drive) has much more than 50 million. A quick search on public figures for these services will give better numbers.
Edit: Those are Apps for Work numbers, excluding consumers - obviously you can't compare just a subset with a total.
User Interface. Dropbox presented a beautifully clean metaphor for how your files get synced. It doesn't matter if you understand what "the cloud" is. My grandmother understands: "put things in folder, folder available elsewhere".
On the other hand we've got Google Drive. Is GMail using Google drive? Is Google Photos using Google Drive? Why don't folders other people have shared to me not appear in my folder? Why isn't 'Shared with Me' the same as "Mine". Is Google Drive the same thing as Google Docs?
Not to mention - Dropbox really pushed the "Desktop App", while Google Drive pushed the "Web First" approach. Add on referral bonuses, Dropbox had some good solid adoption - well, at least among free users.
I use both extensively - Dropbox Pro for everything personal, and Google Drive for everything at work, and I still get confused about the where I saved/shared files to in Google Drive.
> Google Apps has 50 million users, iCloud has 125 million
That source is pretty old and cites an even older source (it looks like it may be from Google I/O 2012). Searching for "Google Apps has 50 million users" actually gives articles from last year saying they have 50 million education users alone, so I'd say the number is a pretty extreme lowshot now.
But more importantly, why would you compare Google Apps and iCloud? The closer thing would be Android backup and iCloud, maybe?