There is always a lot of mystery around why one product succeeds and another, pretty much identical one fails. In a very honest post Isaac Hall, founder of Recurly.com sets out to explain. His product pre-dated Dropbox but ultimately failed to be anything like as widely known or successful.
The main reason for this boils down to overcomplication. As he says:

If you’re starting a new company, the best thing you can do is keep your feature set small and focused. Do one thing as best as you possibly can. Your users will beg and beg for more functionality. They will tell you their problems and ask you to fix it. My philosophy is that they’re right if their feature request is right only if it works for 80% of your customers. Until you have a lot of resources, stay focused on your core competency.

 Of course there were some other clever things Dropbox did, which Isaac mentions:

• closed beta – creates pent up demand while allowing the company to scale the back end to cope
• having a Mac client – many of the commentators are on Macs even if PCs still dominate the world
• creating an early product video which created viral buzz around the product