I think the lack of excuses (that we've heard) is also a cultural thing. (The anthropology major in me says)
In Japan, they have a very collectivist culture that values hard work. If you don't succeed, you're expected to say it's because you didn't work hard enough or do your best, so next time, you'll just try harder or do better. Of course, if there is something wrong (like an injury), you can announce it, but it's not an excuse to stop working hard (even if working hard means working to recover quickly).
In Russia, skating has a culture of competition. If you fail, you fail. It doesn't matter why. When you fail, you have to fight your way to the top - no excuses necessary. When (if) you get back up there, it doesn't matter why you failed in the past, just that you keep fighting for and getting that good placement. If you never get back to the top, oh well.
I don't really have an explanation for America, but if I were to theorize it, I think it would be our tendency towards optimism. We want to see our skaters do well, and most Americans like a scrappy underdog or a comeback. It's good to see a rising star come up (hope for the future), but we also value seeing our favorites do well - we don't like to see them fail. If we hear that the skater didn't work hard enough, we take it as a sign of bad character, not as a promise from the athlete to work harder towards success. Instead, we like to hear excuses. They give us a cushion - not a "well, maybe she'll never win," but rather a "well, she might win once she overcomes that injury." Or, in Karen's situation, instead of "she's can't put it together in international competition," we get "she has boot problems" or "she's just getting used to the Senior level."
Excuses can be good for the narrative some fans want (particularly those every-4-years fans and the NBC-Olympics commentary that feeds them who want hope the US can get a medal). Excuses can be bad for the narrative other fans want (X skater sucks!). Similarly, some skaters can use excuses to their advantage (as a coping tool for a bad result) and others find themselves weighted down by excuses (believing the excuse and not understanding why fixing it doesn't fix their problems).
Honestly, I don't think excuses are a great thing - I hate them - but it's much harder to change culture, so I live with them.