At least in the case of Harry Potter specifically, there's actually a few things that contributed to its success outside of it having a traditionally successful Real American Hero™.

First off, we need to remember that it was cribbing from a lot of other "kid goes to magic boarding school" books out there. The difference in sales success is down to the fact that JKR got a better US publisher. Scholastic has an unfair advantage in the young adult and graded reader markets called the "Scholastic Book Fair". Basically, it's a travelling bookstore event set up in US schools where they sell kids books. If you wanted to start a YA phenomenon in the US, especially back in the 2000s, that was the perfect way to do it.

For similar reasons, Bone outsold a good chunk of other western comics purely because of the fact that Scholastic was the only company willing to touch it.

Another factor is that its obvious Britishisms come across as fantastical to American audiences. I mean, who in America even knows what a boarding school is? This is the same reason why Naruto did so well in America, even though most of the things that seem unique about its world are just fantastic versions of bog-standard ninja tropes.

[0] This is the same reason why Naruto arguably did better in America than in its native Japan.