Because it was cheaper? A standard part used on these kind of systems avalible cheaper than a symmetrical part of about the same size? That is my thoughts. The other is possible reason is 'convention'. "All" ignition systems get driven by a asymmetrical drive. So some new engineer not familiar with airheads designed this... 'New engineers' get the jobs others consider simple, uninteresting ... in electronics systems that is the power supply. What fails most frequently is electronic equipment? The power supply... Makes no never mind .. 'we' will not be installing some thing different.