First is the price. If you need more space than the free option provides, the cheapest upgrade is $100/year. The competition is about $25/year. The $100 doesn't even contain basic features like full text search when searching your Dropbox folder in the browser. You need to pay more for that. I'm sure most companies don't mind paying the extra cost for a better service, but individual users are much more likely to go with the significantly cheaper option even if that means inferior product.
I totally understand that for the big tech companies it's easier to lower prices and even lose money to gain market share. Dropbox will need to find a way to fight back.