You were using two safety items designed to break/release in an emergency situation: the halter and the panic snap. When you have multiple points of potential breakage, one is usually going to give before the other. Perhaps the angle on the halter wasn't right to create the required force to break the leather as others said, or perhaps the snap just has a lower breaking point.
The horse is a mirror to your soul. Sometimes you might not like what you see. Sometimes you will. - Buck Brannaman
"Nothing forced can ever be beautiful." - Xenophon