Why is it so rare in the US to get kidney bean poisoning?
As the question you linked to says, canned beans are cooked thoroughly, so there's no issue. Pretty much the point of canned food is that it's ready to eat; if something required further cooking it would absolutely say on the label. (And given that people eat canned kidney beans without thorough extra cooking all the time, including in salads, clearly they're safe.) Ready-to-eat canned goods are presumably far more popular than anything else, so, most people aren't at risk at all.
It's also not a terribly likely issue with dried beans; it takes ten minutes at boiling to make them safe. Most people end up boiling when they cook beans, since it's usually easier to boil than it is to manage to hold a temperature below boiling that's still sufficient to cook. You have to deliberately get your stove to the right setting to hold below boiling, while if you err on the high side it'll boil. And seeing boiling liquid is a natural indicator of &quot;this is cooking&quot; that a lot of people will naively look for.
The main source of issues is slow cookers; warnings say not to use them at all, because they don't get hot enough, but in fact a lot of slow cookers do end up at a low boil anyway, so a slow cooker isn't actually a guarantee of poisoning even for someone unaware. (This isn't a guarantee, please don't try it!)
So kidney bean poisoning is a combination of several unlikely things: someone has to decide to cook dry beans themselves, they have to pick kidney beans over more popular varieties, they have to have not been warned, they have to have a slow cooker, it has to be one that doesn't end up boiling, and they have to decide to use it. Estimate what you like for the odds, but it's going to come out a lot lower than the 1/50 you guessed in your question.
As for how likely it is, the Bad Bug Book doesn't actually have specifics:

Reports of this syndrome in the United States are anecdotal and have not been formally published.
Several outbreaks have been associated with beans cooked in slow cookers
(i.e., countertop appliances that cook foods at low temperatures for several hours) or in casseroles that had not reached an internal temperature high enough to destroy the glycoprotein lectin.

So it does happen, just not terribly frequently, and it may be underreported since the symptoms are pretty generic.