I think it has to do with the demographics; a lot more poor people where there is such population density. So that reduces the per capita.
Another demographic: many urban types don't even own motor vehicles, esp. in places like NYC where it can be a hassle. In the 3rd world, 2 wheels (motor or not), becomes The Way!!! to get somewhere.
For my generation was much the same as the "soccer mom" hadn't been invented yet.My parents didn't buy a car until I was about 8 years old. My wifes family was much the same-both of us had grandparents that did have a car. One classmate had a Cushman in Jr high(red boxy scoot model, not the racy Eagle!) and never appealed to me as I could easily outrun him with my 3 speed "English bike"! Another guy from the "ducktail crowd" had a HD Aermacchi which DID!!!!!!!!!!!! catch my eye!!!!!!!!!!!!!!! I lived a few blocks from the dingy HD dealer in Topeka,KS and sat on one, when I could until the crochety owner ran us off...
Another demo. factor is age-Lots of bikes in places like FL,AZ & CA where many retired & aged folks live but they don't often ride a MC on nursing home outings, more like a van to Walmart or the park?
Ebay auctions will also show the "other demopgraphic", i.e., the fact that many own a bike, yet never ride it.