In the US, I think that ppl are so used to he disfunctional family plot that the automatically dismiss any show with it (also the case w/ Arrested Development). It takes a few watchings (sometimes forced) before one realizes how good MITM is. Although maybe that's just b/c the 1st show I saw was Houseboat and it took a few episodes to get over that. In other countries, shows surrounding disfunctional families, although still present, aren't as common. So, unusual shows such as MITM become really popular.
Also, compared to other exported US shows, such as Friends, Drew Carey and Will & Grace, MITM is child friendly. This might be a factor.
I may be way off. My only experience in non-US comedy shows is with India and '80s Germany (German class), both of which are pathetic. And of course classics such as Monty Python and Fawlty Towers.