Because motorcycling is a broad church and the Rocket satisfies but one small niche within it. The standard rocket does a few things very well and many things OK.
It goes very well off the mark but there are plenty of other bikes that go better. It corners well considering it's weight and size but most other bikes corner better, something I learned chasing a mob of GSXRs, RSVs, CBRs and KTMs a couple of weekends ago. Through the twisties? Just forget it.
The only thing where the Rocket is actually the leader is for cubic inches and torque. Which really only gives you bragging rights and is still handicapped by the weight of the thing.
Practical and logical reasons aside, what it really boils down to is whether or not the Rocket is "your bag, baby" ... and that is entirely subjective.
The Rocket IS my bag, baby ... but I'm not everybody.
What the Rocket had that sealed the deal for me is:
Effortless torque.
Comfort - Not perfect but it's getting there.
Pillion comfort - My Mrs found the CBR1100XX legroom too cramped.
Low to the ground - My Mrs did not like being perched up high on the back seat of my CBR1100XX, especially in corners.
Looks - Yeah, subjective but I actually like the look of the thing. It's mean and purposeful.
Luggage capacity - It's not the best but it's enough for me.
Price - The other bike that would do all of this would be a Goldwing. But I was never a fan of the look of the old ones and couldn't afford a new one.
The Rocket ticked enough boxes for what I was after and I'm well happy with it.