It's real simple, at least to me. They are not affordable for the young generation. Since they alienated people like me (when i was young) with their extraordinary prices. My kids never saw one parked in the garage, I was never able to give them their first ride on one. I can buy 2 of anything else for the price of one HD. Sure there are some cheaper versions like the sportster, but I didn't want that. I wanted a Harley when I bought my shadow, I also wanted one before I bought my rocket. Now I'm over it and I've lost all desire to ever own one.
People got into the Harleys back in the day because they could afford it. It's just not the case for the average working man anymore. They dream about it until they wake up and see something like a rocket with 2x the power for a 1/3 of the price.