I have wondered the same thing. I did not find the Rocket till a few years ago and now I’m 61 years old so I guess I’m the typical old man on a a Rocket..
So why? When I saw the Roadster for the first time I thought it was a bit bulky and frankly ugly. After I rode it I fell in love with the torque and trade my hoped up Harley. It was my fast bike.
So I think the reason is this. After years of owning different bikes a guy just looks for the more unique. A real big boy bike that says I’ve done all that other stuff and have found the top of the cruiser heap. For those that have gone the sport bike route have raced enough and lived on the edge long enough to realize that this could get me into trouble. As we age we desire a bit more comfort but don’t want to ride a weak bike.
It takes a while for guys to appreciate a Rocket. Still fast, very comfortable and in no way sacrificing the big power. It’s not a guys first bike for sure.