Wing Chun is an internal, chi kung art. All its principles, stances, techniques and philosophies point to this. Any who know chi kung will know the answer. Any martial system is much easier to teach without the seemingly mystical internal side included. Martial arts have become very commercial.
Teachers try to attract more students to bigger schools so they can make more money.
Students are pushed through the training quickly, and thus they do not develop quality skills. Teaching chi kung properly requires a dedicated patient student and a wise teacher who can point the way. Those who still attempt to teach the internal side find that it is most difficult to teach. All a good teacher can do is point the way, he can invite the student to experience his chi, it is up to the student to learn it. It is much easier to teach a technique or a motion.