It’s mainly because it’s the single most supported “cloud” service. You can almost guarantee that any ap or program is going to be able to use it without jumping through hoops, and that’s vital when you need quick, reliable off site storage.
Out here in the country, I might have seconds to back up what I’m working on if weather gets crazy fast. It isn’t often, and even rarer that it would also cause problems with my own storage too, but I’ve lost an entire almost finished novel to a lightning strike before, so I’m a tad paranoid.