Because syncing a monolithic binary file is trickier and more intensive than syncing a bunch of little files, once your repo is big enough. If your git repo was corrupted, you might recover some data. If you corrupt a binary database, good luck with that. Dropbox is also harvesting your data for various purposes. I hope your code isn't valuable, because it could be leaked or stolen by employees of Dropbox. There's also something to be said about using what other people use instead of bizarre niche tools.

Disclaimer: I have used Fossil and it's a decent system for small applications where everyone is on board. But I would not recommend it due to the reasons above.