That's weird that it's not very popular in the US or Canada, because from what I've been told, it's very big in Mexico--so big that they actually have BG's based in Mexico, and two characters from that region (to put that into perspective, I know of only one other game series with Mexican characters--Tekken)--Ramon and Angel.
Personally, in regard to MK, I think the series peaked at MK2. Mainly because it had Jax, one of the coolest characters ever in a MK game (The guy did the Riki-Oh head squash... he 0wnZ j00 by default <IMG SRC="smilies/biggrin.gif" border="0">). Then, it just got worse with lazy character design than any Capcom game can ever be accused of being (5 karate characters? Try a grand total of 14 ninjas in Mortal Kombat Trilogy). If it had been a 3D game, the block button would have made more sense, as the changing camera angle would make determining which direction is "back" nearly impossible.
Then again, every fighting game series seems to peak at 2. It's the Rule of Twos. The Rule of Threes, of course, is that most of the cast from the previous one is scuttled in favor of "new blood." Exceptions: MK3, SFA3