Perhaps because Korihor defended his actions as the result of the devil being in the form of an angel who lied to him, he hoped that Alma would lift the curse. Alma declines. Korihor had admitted to being a liar, and not only a liar in small things, but also with respect to essential elements of Nephite politics and religion. He had been influential, and, therefore, dangerous.
As a liar, Alma could not trust him to cease lying. It is also likely that the very obvious punishment of being struck dumb would stand as a permanent declaration that he had indeed lied, and that that would begin to heal any of the damage that he might have caused. Korihor had been clearly given ample opportunity, even the opportunity to reverse his lies, knowing what the sign would be. He didn’t, and Alma saw no reason to reverse the curse.