It actually isn't a lot higher for bikes sold through bike shops. When I said less than 10% overall, I was being a bit generous, the number from all bike outlets is actually less than 5%. From bike shops it runs around 10%-12% on average. If a shop specializes in high end bikes then it could be much higher. But as the average bike sold through a bike shop sells for around $400-$450, that isn't a lot of high-end bikes.

Even for most bike shops, their bread & butter bike is the hybrid, next comes mountain bikes.

I think the current figure for the number of people in the USA who ride a bike at least 6 times a year is 43 million. Even out of this group, which excludes all of the people who own bikes but almost never ride them, the percentage riding drop bar road bikes is low. I'd guess maybe 3-4 million of these riders are on drop bars.

For example, I work at the University of Wisconsin in the bike friendly city of Madison. Thousands of students ride to campus on bikes. But when I walk past the bike racks only about 5% of them have drop bars.

Now if you limit this population to say those who ride a bike on 50 mile rides at least three times a year, then the pct on drop bars would go way up. Although I think that pct would still be fairly low if the criteria were 20 mile rides. A lot of people take 20 mile rides on trails with hybrids and mountain bikes.