Because Creon knows Antigone is right and he fears that Ismene will only confirm her position: that their brother, Polynices, deserves a proper burial. Creon also knows that the sisters have the rights to his throne and thus pose a real threat to his ill-gotten crown.
Moreover, Creon, the chauvinst, is not about to let a woman push him around. He already feels somewhat emasculated by Antigone's strong argument, In lines 541-542, he has complains to theleader: "I am not the man, not now: she is the man / if this victory goes to her and she goes free."
Just before Ismene is brought before him as well, he snaps at Antigone, "Go down below and love, / if love you must, love the dead!" While I'm alive, / no woman is going to lord it over me" (592-594).