How come most of the time when helicopters went up against fixed wing aircrafts they usually get clobbered?

There was a study regarding rotary wings versus fixed wings in aerial combat throughout various conflicts around the world since the 50's. The performance of helicopters were quite dismal when they faced "armed airplanes" airplanes which weren't fighter planes. During Vietnam War American helicopters were blasted out of the sky by North Vietnamese Migs. In the Falkland War a British helicopter got shot down by an Argentine Pucara. When Iraq invaded Kuwait in 1991 their helicopters were badly mauled by armed Kuwaiti Alpha Jet trainers. In the first gulf war an Iraqi helicopter was shot down by an American A10 using a "laser guided bomb", not even a missile. What is the point of arming them with air to air missiles if they are not going to be able to defend themselves anyway.