According to Steven here are three reasons why this might be the case.

Firstly, there is a perception that breaststroke is an easier stroke so many people approach it in a more gentle way, enjoying the sense of gliding through the water. However, people have the mindset that crawl is hard work so they apply too much effort to all of the movements wasting energy with arms and legs. By the time it gets to the point in which they need to inhale, they feel exhausted. In a Shaw Method workshop, you will learn the secret of smart swimming –  to differentiate between non-propulsive and propulsive movements which will make you crawl much more sustainable.

Secondly, the crawl is a rotational action in which the body moves from the centre to the side. The face is much closer to the surface at the point of the in-breath which means that the timing of the breathing needs to be more precise. The Shaw Method teaches you to lead with the head which will place your body in the optimum position to breathe in.

Thirdly, in the breaststroke, you breathe every stroke so the time between strokes is not very long at all. In crawl, you breathe every three or five strokes so the time between breaths is longer. You can however ill learn to achieve greater control of your out breaths so when its time to breathe in you feel more relaxed and therefore more able to inhale passively.