The program itself might not get information efficiently to do conflict resolution (or not at all): for example, you edit a file offline and sync, Dropbox and friends wouldn't be smart enough to just append both of a few bytes worth of data that a password-manager controlled service could since it would be aware of the data structure but would just dump both files, and then both on another conflict etc
So I guess it's just not the same type of sync service that you get for free in those many services
(also I think it's more than a sub-Mb, you have icons there, but also images of docs and what not)
Though maybe this is not an issue as you mention some of the keepass-based apps that go the "app-sync" route instead of manually placed file?