If I'm not mistaken the last industry specific report indicated new motorcycle purchases were down over 50% since the peak recorded in 2006 and it hasn't rebounded. I'm curious as to specifics of such, alas, I'll leave that task for the analysts.
I can't speak to specifics, but I know that there are a few contributors to this:
1. Younger generations are facing a FAR more difficult economy than older folks - when you're paying upwards of 60% of your paycheck on rent alone, spending big bucks on a toy and gear isn't high on your priority list.
2. If you buy a ~2005 or newer motorcycle, you aren't going to see a massive jump in performance compared to a modern bike of the same caliber. In my case, I could never justify spending $18,000 on a New S1000RR that's only a couple seconds faster per lap than my 2006 GSXR that I bought for $4000. The used market is still booming.
3. In Harley Davidson's case, they're aging out too - they cultivated this "badass biker lifestyle" which is rapidly losing it's popularity, especially among younger generations. What's more, their bikes are dated, notoriously unreliable, and very expensive, which is why they're experiencing a huge fall in marketshare.
4. There's more vehicles on the road than ever, and motorcycle saftey statistics haven't improved much over the years. Not a lot of people want to take that kind of risk anymore.
When you put it all together, it's no surprise that sales numbers are down - they're probably going to get worse from here too, unless we get our economy in check.