Probably because Yang Cheng Fu was the first master to go public. Chen is reputed to be the most physically demanding style. Sun is thought to be the least physically demanding style. Tai Chi is taught in the classes I am attending in several distinct sections. First the form.
All martial arts have forms.
A form is a set pattern of movements which is repeated over and over again by beginner and expert alike. The forms contain all the fighting techniques of the style, however, it appears that it requires years to learn what each movement in the form means and how it could be applied. Grandmaster Chien-Liang Huang taught a 112 movement Yang style form. Each teacher assembled the pieces into a form that accomplished whatever it was that he was trying to accomplish. Many people practice the form only for health and stress relief without learning Tai Chi as a martial art.