There's a good reason: I would argue that Dropbox has questionable security practices (by that I mean look around their site and try to find an in-depth description of how then insure individual account security... it leaves me with many questions :-). Dropbox stores and manages the encryption keys on their servers and they don't let users specify their own encryption keys. If someone manages to break into their server farm and gets access to those keys (or the method by which they are algorithmically generated), then lookout!

At the same time, a lot of people email sensitive documents into and out of enterprises without any sort of encryption. I'm not sure which is worse.

(no, I'm not a enterprise IT guy :-)