The Dropbox file UI side of things is optional for users; they have to seek it out, either on the website or by having one of the Dropbox apps - there’s nothing stopping you from having a Dropbox account purely for syncing data, without ever installing the Mac app or viewing a directory on the web site. But their syncing of files works. Apps can build better UI on those files whether they’re stored locally, stored in Dropbox, or stored in iCloud. But Dropbox has proven it’s reliability, and iCloud hasn’t.
Rene Ritchie says that “iCloud is the right idea still not realized, Dropbox is the wrong thing done brilliantly well.” Instead, I think that it’s not yet clear whether iCloud is the right idea; the current feature set is inadequate, and we don’t know in which direction Apple will take it. I would liken Dropbox to Unix. It’s not the pure, modern system that one would design from scratch today. But the plumbing works, and a good user experience can be built on top.