Funny how it will always be "American mythology" when actually that specific thinking pattern is not part of Roddenberry's vision, and when a country like Germany, which had been Nazi hell only roughly 65 years ago, now have a woman and a gay dude in charge. I wonder how long it will take for Americans to be that extremely liberal.
But maybe it's because the Americans dream about being liberal like in Star Trek, while the foreign nations simply just do it.
You have to look what Star Trek actually is. It's imperialism vindicated. It's Americans going round space telling people how to act, how to live, and that's fine: I enjoy it.
You are right, it is liberal mythology, taken from America in the 60s, and not really indulgent to the resurgence of Conservative christianity, at least on a mass appeal, in today's America. But truly, the mythology is only perceived, as a whole, by those foreign markets, so they can reject it.
You can also blame the foreign markets, Europeans were not always unwilling to indulge a Star Trek picture, it's just lately, that many European countries have had somewhat of a cultural crisis and have become small-minded, unwilling to accept anything that will invalidate their increasingly precarious way of life. Same goes for South America and to a lesser extent, Japan.(Though I think it did okay there?)
And I really wouldn't give "Roddenberry's vision," veneration like that, considering that a pure Roddeberryesque vision, almost tanked TNG and was thrown out of the airlock, movie-wise, much earlier. Berman's unwillingness to deviate from it, gave us a bland, stately Star Trek that almost killed the entire franchise. However, at least it was vindication and held onto it's idea for a long time, even after his death.(Though I would argue the massive dark left-turn at the end(Nemesis and Enterprise Season 3), turned the knife.)
Bit of a long post :P
EDIT: Actually I think Roddenberry managed to exclude any subtext of imperialism from the earlier seasons of TNG, believe or not, I think Jeri Taylor(you wouldn't expect, no) was the one who slight etched it towards a more imperialist angle (Insert Voyager crewmember challenging Bland, backwards culture.)