Do universe 6 saiyans transform more easily than universe 7 saiyans?
Officially the original super saiyan transformation was triggered by pure anger and enough raw power to transform. The only possible explanation this is that U6 saiyan have the power but they are not that interested in fighting so they never really explored their power.

One more thing that you can't expect a story to be perfect and with no plot armor because the mangaka has to make the story interesting.