There are two factors here, and they are not mutually exclusive. Both have been discussed, at least on the old forums.
Europeans, and even more so, Japanese, exhibit a higher level of culture and specialization than do Americans. Primary factors (according to the social experts) are both linked to urbanization: (1) lack of outdoor and other activities turn focus into things that can be done/ collected indoors, particularly small dwellings, and (2) expendable cash typical of urban systems (i.e., much higher income).
It's not that Americans can't (and some very much do) become highly specialized equals to their EU counterparts, but they have multiple outlets to spend their cash (motorhomes, American football, five televisions, etc.) On another scale, why are there no specialists in Leps in Vanuatu? Because for the most part they have neither the time nor the money to conduct such research; a $100 book is nothing to an American, a $100 to most in Vanuatu is a week's or month's pay.
And, so far as entomology is concerned, yes, EU (and very much so Japan) is more lax in enforcement of laws concerning Lep-related controls. With so many restrictions, compounded by blind enforcement, on US activities, it's no wonder US amateur entomology has lost the interest of the public. And not just entomology- an average tropical fish store in Germany beats even the best stores in USA. And on, and on.
There is though, I believe, still a dramatic difference in US vs. EU & Japan perspective: Give a Japanese $500 to buy Leps, he'll buy ONE that's special; an American will buy 100 specimens at $5 each. That's the McDonald's mentality: more beats better.
Still, all of that is excuses. There is nothing STOPPING any American from purchasing rarities- just have to part with the cash. But one cannot be both cheap and generic, then complain about selection. The market supplies what the the market is willing to pay for.