[KMag]

I suspect there are a few factors:

* Motorcycles aren't very dangerous to bystanders.

* Approximately zero infants or toddlers are routinely transported via motorcycle.

* Motorcycles don't make up a large percentage of traffic in the U.S.

* Motorcycle deaths are more likely to leave donable organs intact, so it's a bit like the lottery, but for organs. Some people opt in to entertainment, and the rest of society benefits.