Because, as far as I know (I haven't used TrueCrypt too much), it will see the volume as a file and then you'll be syncing a huge, x GB file every time you make the smallest change within it.
No, the crypto in TrueCrypt doesn't amplify the number of changed blocks. But it does disperse the changed blocks throughout the encrypted image, and this does result in higher bandwidth used during the sync (at least with DropBox). Here's an analysis I did a few years ago: