Nobody can seem to do it very well, despite all of them having one. Every major provider has some sort of cloud drive product, but all of them, save for Dropbox, have some annoying limitation.
Apple iCloud Drive: Unusable out of MacOS.
Amazon: Unclear storage limits with potentially catastrophic consequences for going over them. No free tier. No Linux client.
Microsoft OneDrive: Individual file size limits. No Linux client.
Google Drive: Stories of unreliable syncing behavior, app instability, and so forth. No Linux client. Google history of killing non-core products.
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Not only did Dropbox get there first, they did it mostly right, to the point where their competitors can be described as also-rans.
The other thing is that Dropbox has "good file sync service" as their entire mission, while every other provider of note has it as some value-add into their other services, meaning it's subject to the whims of what those companies are doing on a particular day.
Also, Dropbox is the only one that offers delta-sync which is a godsend when editing multi-gigabyte PSDs/Media Files. Other providers would simply not work for this use-case with my 100KB/s upload speed. Seems very wasteful as well, uploading every file from scratch for every edit.. kinda curious how users on these providers deal with it?
For iCloud, you actually can use iCloud Drive via a web interface from other OSes - I think the biggest limitation is actually that you can't share arbitrary files, only certain types of files (Pages, Keynote, Numbers, Notes, Photos).
I could easily go all in on iCloud Drive if I could make arbitrary files available to other users.
Apple also has a Windows client for iCloud Drive, and you can use standard Word/Excel/PowerPoint files and still edit them through Pages/Numbers/Keynote on a different Mac.
And that's fine for you and me, but limits its adoption (hence appeal, hence longevity) to people that download unofficial, unsupported third party clients and manually set up cron jobs.
The reason Dropbox is kicking ass is that nobody wants to deal with the manual work. Download a program, put stuff in folder, stuff appears elsewhere. No thought required. Simple.
I've been using Dropbox on Linux for many years, and it's definitely had some occasional issues with memory leaks, and its general CPU and memory usage has definitely increased. Used to be it never went over 100 MB RSS, but now it's always well over that, at about 250 right now.
Anyway, I run it at nice 5 and "ionice -c3", and even though I'd like to switch to something else, I still haven't found anything better for Linux use. Everything I've looked at has some issue that makes it a non-starter.
Apple iCloud Drive: Unusable out of MacOS.
Amazon: Unclear storage limits with potentially catastrophic consequences for going over them. No free tier. No Linux client.
Microsoft OneDrive: Individual file size limits. No Linux client.
Google Drive: Stories of unreliable syncing behavior, app instability, and so forth. No Linux client. Google history of killing non-core products.
--
Not only did Dropbox get there first, they did it mostly right, to the point where their competitors can be described as also-rans.
The other thing is that Dropbox has "good file sync service" as their entire mission, while every other provider of note has it as some value-add into their other services, meaning it's subject to the whims of what those companies are doing on a particular day.