The reason is very simple. Look closely at the lists that attended each. There a 3 to 5 top notch eastern high academics (3 of which are Ivy) that, for reasons I am not aware of and don't care about, do not (or did not in certain years) attend HF but do attend Stanford. If your son is not targeting those schools - not sure of exact list but Penn, Brown, Cornell, Georgetown come to mind - or they are not high on his list, then there likely is not good reason. Also, more exposure is better - what are the chances that each coach that your son wants to see him actually can over 2 days (or in case of a P, about a combined half hour - 2 IP per day)?