Seawatch is right on track. The actual land and distance of the United States is too huge for the same kind of mouse fancy, I think. It's comparable in size to Australia, another country that has difficulty in keeping a mouse fancy together.
The AFRMA is our oldest club (existing since the 1980s, if I remember correctly) and it is our best-funded, most professional, and most active (due to the fact that it's in a huge population area--Los Angeles). As an example, from me to the AFRMA is 2200 miles. According to Google, this is about the same distance between London, England and Jerusalem, Israel.
And I live in the middle of the country! If you lived further north or east, such as folks in Maine, you'd have to travel around 3500 miles unless you wanted to take the hassle to move between countries (through Canada), and with live animals that brings even more challenges. Considering the difficulty and expense of taking mice on planes, that's a five or six day trip by the time you stop to sleep and eat, and a five or six day trip back, so in practical purposes you're talking about a two week trip to show mice.
The ECMA solved this problem by moving locations every few months, but that means our shows are sparse and far-between because we have to be in one state in March and another in October/November, and our attendees are rarely the same (Jenny (WNT) and/or I are the only people who go to every single one) because people can't travel across the country every few months. We operate on an all-volunteer skeleton crew because like nearly all small animal clubs in the US we're so underfunded and paid for by one or two individuals. Member fees do help but unless you have a million members, traveling such long distances is super expensive.
I wouldn't be surprised if in fact we had similar percentages of people who are interested in mice as does the UK. What's different is that our country is enormous and we're super spread out. Even in NYC or LA (the two largest population centers), mouse people are still spread out with maybe three or four people who are serious fanciers. AFRMA shows rarely have more than half a dozen attendees. That's the single biggest issue in getting the American mouse fancy more developed, I think--geography.
Honestly, as an American who culls heavily, inbreeds heavily, and tells people not to breed petstore mice, I am looked at as a mean old a nutter, and I do have enemies because of it! I have a feeling that in the UK this would all be normal.
All this is why I would love to move to England. Any English citizens wanna marry me (male or female, I don't care)?