Depending on the theme this could happen. It's not great coding, however I have seen that happen. Did you restore to the exact theme though or did you make changes (even a name change will cause reverting to defaults)?
It may be as easy as once you go activate the original theme, go into it's settings page and click save.
Some themes in their setup section will have a back up and restore option. If you previously backed up the settings you can restore them there.
If you don't have any of the theme backed up, you can always do an actual database restoration from before the switch and this will revert the changes back to their working state.