Because dropbox will save bandwidth by only updating the parts of files that have changed. If you use truecrypt the entire file will change for every small change you do. Basically you lose any advantage of using dropbox over a standard FTP system.
If it's only a KeePass database or a small (1MB?) TrueCrypt volume for text/documents, the traffic still won't ever become appreciable. Also, you're implying that the only selling point for Dropbox is that it uses deltas, which is far from true.
Actually dropbox and truecrypt work really well together, even with large truecrypt containers because of the way truecrypt updates its container on file changes.
