[I always knew the US market was a minor player, but I didn't know it was that minor]
It wasn't always that way because in the mid-60s to early-80s, I'd guess the USA sales numbers for Honda were 40-50% of worldwide totals, perhaps even higher. If you owned a USA Honda dealership in 1966-76, it was like having your own money printing press! Several things happened between then and now to cause the change.
One factor was Honda building factories in other countries to service that region or country. The factory in Brasil actually predates the US factory by one year and was up and running in 1977. I'm not sure about the launch date of the factory in Belgium but it existed in the mid-70s and they teamed with Montesa in Spain in the early-80s to get into the Euro market. I believe all of our beloved 2010-11 NTs came from Barcelona. They also paired with Hero in India but around 2013 that co-op ended with Hero going its own way. And there is a fully functional factory in Thailand that has been building Honda cars & bikes since the mid-80s. That's about the same time the factory in Mexico was built.
I would say that overall the traditional motorcycle market in the US is gradually disappearing as X & Y gen kids seem to have no interest in motorcycle or even cars. Conversely, "developing countries" are still scooping up basic transportation models (50-150 cc) as fast as they come off the assembly line. People here complain about our high gas prices but they have no idea what it costs abroad where people make much less than we do: $6-to-$8 per gallon has been common for years now.
In the US, the disinterest by X & Y gen is worrisome to all motor vehicle makers but especially H-D which is one reason they may have developed the lower cost 500/750 V-twins but so far those models are almost invisible. Trying to attract those young people raised on electronic toys is the major reason all the car makers are adding lots of touchscreen features - most of which only cause trouble!
Of all the makers, Yamaha has probably done the most to bite into Honda US market share with lots of new, innovative models being sold for reasonable dollars but the sales bite is only a guess on my part. Honda lost sales but perhaps not money by not getting into the adventure bike segment earlier so we'll have to see how that turns out with their new Africa Twin. People said the same thing ("too late") about the Odyssey mini-van and it is and has been on top of the minivan world in the US for many years.