Yoga may help relieve asthma symptoms

Why was not there control groups with other kind of breathing exercises? It seems very clear that the effect was minimal and close to non-existent and limited to the breathing training, so its extremely likely that Yoga had nothing to do with it. Refusing to include other breathing exercises seems very suspicious, almost as if they may had found a bigger effect if they did and the Yoga part of the study would then become irrelevant.