From what I see and hear about being your kids' taxi until they get a driver's license (at 16yo already), I'd assume the difference is pedestrians. I'm nearly 30 and never owned a car, coming from a village and living in a town. Shops are walking distance, I take the bus to work (non-covid times) for environmental reasons, and I think a small majority of people I know do the same (most of my friends/acquaintances are in NL).
Germany is already quite car-centric by my standards, yet they have the most contributors. There's still a lot of pedestrian traffic for the short distances inside of towns and cities, just no medium-distance cycling traffic. So why do they have the most contributors?
Living in Germany, the mentality is different. I have been trying unsuccessfully for three years to find a good definition or concise explanation. There's something that drives use of Threema (paid), PGP and Linux (relatively hard to use), and OpenStreetMap (when sugar daddy Google gives you free maps already). Certainly Google's map here is worse than in their home market, but it's not bad either.
Everyone in Germany will tell you that by the end of secondary education, they're just so done with the whole hitler thing. It's a huge topic throughout the educational system. So I guess morals and things like why privacy matters gets ingrained as well? But that doesn't hold for other European countries, in NL the Linux/Threema/OSM/PGP usage is similar to what I hear on HN.
So a combination of factors, with as biggest common denominator probably pedestrians, plus the mindset in Germany, and maybe a tiny fraction the cyclists (which is literally everyone in NL, by the way, it's not a subgroup but a state of being, or at least I learned that "cyclists" is a laden term in North America from someone who is from there and moved here, but anyway OSM contributorship in NL isn't that huge).