Before Dropbox, there were already 80+ attempts at doing exactly the same thing by various other companies (Microsoft by itself was responsible for at least eight of them concurrently.), and none of them worked well enough that they were ever adopted by anyone you've heard of. Dropbox is just right, and not for technological reasons, but rather for perfect customer fit.
To put it another way—it takes more people to build a bridge than to craft a samurai sword, but which metalworker brought more experience to bear? Games are regularly pushed out by developers just starting in the industry—all that's required of them is a willingness to be on the bleeding edge of graphics hardware APIs, and precious, precious manhours. Dropbox, on the other hand, could only have been designed by someone with an extremely high aptitude for user interfaces.
To put it another way—it takes more people to build a bridge than to craft a samurai sword, but which metalworker brought more experience to bear? Games are regularly pushed out by developers just starting in the industry—all that's required of them is a willingness to be on the bleeding edge of graphics hardware APIs, and precious, precious manhours. Dropbox, on the other hand, could only have been designed by someone with an extremely high aptitude for user interfaces.