I was actually thinking about this a bit more, and I thought, why doesn't everyone drive motorcycles IRL? What are the drawbacks?
Comfort: Motorcycles are less comfy to drive than cars, especially in inclement weather, both from the driver's position as well as climate control and various amenities.
Safety: Cars are way more safe than motorcycles. Some idiot who isn't paying attention on the road could get you killed, on a bike. In adverse whether, the danger increases exponentially. There are some weather conditions in which you would never ride a motorcycle, making it a completely useless choice as a primary vehicle in some climates.
Cargo and Passanger Capacity: You can haul things and people around in cars, not so much in motorcycles.
Meanwhile, the pros of motorcycles: ease of movement through traffic, fuel economy, cool factor.
In RL, motorcycles are a poor choice for a primary vehicle if you can afford a car, which many people can with financing.
Anyway, where I'm going is this: in SD, the comfort/safety/cargo issues are way less significant than in RL, which removes some of the natural real-world barriers people have to motorcycle ownership and makes them that much more attractive in game than they are in RL. Additionally, the pros of motorcycles seem well represented in the game.