I pretty much guarantee this was an accounting-based decision. They weighed income from users using these very rare file systems vs the cost of maintaining support and found that it wasn't worth it.
It's not as if these users can't just create a separate partition just for Dropbox or use Dropboxes web UI. I use Dropbox on a regular basis and never installed the app. Anyone paranoid enough to run a fully encrypted FS wouldn't want to store sensitive data in an online file locker. And anyone with a real reason to run a fully encrypted file system wouldn't use Dropbox on the same system. I imagine the number of effected users is very low, but probably also very vocal.
Biting the hand that feeds IT
