Why didn't Jesus condemn stoning altogether instead of saying "He who is without sin...?"?

Because God has to find the perfect balance between justice and mercy. The two taken together shows that a person who does wrong deserves to die but will be shown mercy. The right kind of people accept with gratitude and change their lives for the better, like an alcohol abuser who is given a liver transplant he doesn't deserve and vows to stay off alcohol forever. The wrong kind of person scoffs at having "gotten away with it" and messes up even worse, like the gambler who gets in deep with a loan shark and someone bails him out but instead of sorting his life out gets into even deeper debt again expecting someone to bail him out but winds up at the bottom of a river wearing concrete shoes.