Why are my menu bars always on my left monitor?
It depends entirely on one pref...

System prefs > Mission Control > Displays have separate Spaces.



This will, however, completely change other aspects of your workflow.
The dock will always be present on each screen; you can no longer use screens in pairs, each is independent, so changing Spaces on one screen doesn't take the other with it; you can't have the same Space on both screens at once, but you can invert the left/right order of Spaces by swapping each on the other screen, via a 3rd Space.

Personally, as you can probably tell, I hate it ;-)