Why do MMA fighters do not seem to have much success when they compete in professional boxing?

There is always talk about MMA overtaking boxing and how boxers can not compete with the mixed martial arts because MMA trains fighters in an all-around fighting game: grappling, punching, chokes, submissions, etc. But how come there is no discussion of MMA fighters who cross over into the professional boxing world and they usually have no success in boxing? Don Frye is a two-time UFC Champion but when he tried boxing he suffered a KO defeat and quit in his second pro fight. Anderson Silva quit after only two fights and long-time UFC veteran Patrick Smith, who competed in the inaugural UFC and was a UFC II finalist, has fought in sixteen professional boxing matches and lost eleven of them. And he has been knocked out only eleven times.