I don't think it's superiority of English humor, or rather, British humor, over American humor, but more a question that American TV and movie producers are looking for hits, top sellers, and content is only a secondary consideration. So, I think they see a show that is highly rated in another country, so it could charge higher advertizing rates, and its content takes a back seat.
Some shows that were copied have universal appeal, in the English-speaking world, at least, and succeeded. Look at "All in the Family", for example, or "Sanford & Son". Except for the reference to the original works in those shows' closing credits, there's nothing British about them. What's British about a family father and his interaction with his wife, his beloved daughter, and her slacker sponging husband, for example?
"The Office" is another example of a show whose basic plot is neither British nor American, and so, can appeal to audiences inside and outside the US or Great Britain. (Though I think it has jumped the shark, and I wish that its executive producers would end the show, with a logical conclusion based on the plot, rather than to let it spin on and on as just another sitcom.)
On the other hand, the more recent "Coupling" copy failed dismally in prime time on the mainstream network (while the original has a decent following on BBC-America and on public television).
And "Kath and Kim" appears to be on thin ice. It has a talented cast, but it's timeslot probably doesn't help. It's the lead-in to "The Office", and it probably should follow it. Its ratings are low, and critics are panning the writing. Personally, I've never thought Molly Shannon was funny, whether on "Saturday Night Live" or in any of her movies. And Selma Blair is a good actress, but I think she's out of place as the daughter.
Prosit!
Brad