Don't forget that the Jedi of that time period did defeat the Sith pretty thoroughly in the Great Hyperspace War, forcing them to flee to the outer reaches of the galaxy.
So maybe the Force was balanced. Jedi = Other Sith + Vitiate. But since Vitiate did not participate in the war, the Jedi were much much more powerful than the other Sith (in addition to the Sith infighting), which allowed the Jedi to win.