The thing is, if you take England, pick a venue, probably 95%+ of the population are going to be within a 2-3 hours drive of it, if that. So for national XC, my club used to simply put on a coach, and we would travel to it together (if you chose to).
With the USA, I could imagine there is considerable expense getting to a race if, for e.g. you live in New York, and the race is in San Francisco. Or for pretty much anyone going out of state. There would also be hotels involved. And planes.
The English national XC has a long tradition and it will always pull in runners - a lot of people prefer it to the inter counties (I have always preferred the inter counties). Also, when it's at Parliament Hill, the catchment for that area is brilliant, so the amount of people who are able to run is going to be good. There are millions within an hour of that location.
XC is cost effective. You have coaches with boots full of spikes that they give to youngsters, athletics club fees that are often maybe £20-£30 a year, free kit provided sometimes - they go above and beyond to make sure it is accessible to all - if someone desperately wants to run or has a talent. Pretty much everyone has socks, you can get black shorts from SportsDirect for £1.49 and a club vest can be loaned out. Then you're transported to the race. Point being, it is accessible.
To the second post:
For high school there is district schools, then county schools, then English Schools (the best meet of the year).
For university there was BUSA and now I believe it is BUCS.
I cannot speak for anyone in France, Spain etc but in England, EVERYONE tries XC in high school, it's often a weekly thing. And the high schools then send the best people to the district schools race which is usually in the town the high school is near or in (on a field), and from there you are selected for the county team. So people with talent are picked up. I'd imagine anyone who wants to run in the district schools probably can. If you liked running, you would stand out in your PE lesson, everyone in the school year would have a chance. That is how I ended up in a club, there are coaches from local clubs at the district schools (even though they are not part of the system) and they approach people.
Also for a fair comparison I think you should combine: England + Wales + Scotland + NI championship numbers to make the UK. I'd be interested to see that number.