Good question, young grasshopper. When I first started riding there seemed to be more veteran sportbikers on the road and almost all waved back. This was also a time when chicks flocked to bikes no matter who was on them. Then the pretty boys who got all the tang already started riding sportbikes and if your bike wasn't perfect they wouldn't wave. They don't get that it's an unspoken rule, you see a bike you wave. If they don't wave back, atleast you're not the jerk!
Anyway, I get about the same percentage of waves back from cruisers as sportbikes. I actually have to remind myself not to wave to bikes when I'm driving the truck
I'm frequently surprised by how many stereotypical "hardcore harley" riders wave back, but I attribute it to how ratty my bike looks
