Because he wouldn't have won; Garrosh probably wouldn't have taken the chance. Garrosh had already tried to assassinate him by the point Vol'jin puts the rebellion together, and the last guy who tried to dethrone Garrosh in a Mak'gora died under mysterious circumstances himself.
I think everyone knew that Garrosh was through playing fair at that point in the story. Vol'jin was of the opinion that Thrall would be killed on sight in Orgrimmar, and the same goes for himself tenfold.