Because in principle the files on Dropbox could just be dumped on the open internet -- Dropbox never sees anything unencrcypted. There is a lot more trust in a single tool.
EDIT: No, I'm wrong. I was thinking in terms of login credentials (which neither are able to see); but LastPass does look at the URLs associated with your credentials so they can pull favicons
Because in principle the files on Dropbox could just be dumped on the open internet -- Dropbox never sees anything unencrcypted. There is a lot more trust in a single tool.