I'd say it's mostly climate.
When I lived out west, I owned many bikes but had never owned a car till I moved to michigan at 28. (now 46, and have owned about twice as many bikes as cars)
If you have only a few months of (not always) great riding weather, bikes last longer,(and you get used to them 'sitting' in the garage) but if you have year round riding, they tend to wear out or get wrecked.
In california a low miles bike was unusual, here in michigan it's fairly common to find 20 year old bikes with less than 10000 on them.
A year or two after I moved to michigan, I made four trips to Cal with six bikes on the flatbed and sold them for triple what I paid in michigan, I'm sure they've been ridden to death by now.
Also, in areas where demand is higher, the bikes that do make it through are often handed down to freinds or family so don't make to 'market'
Ken.