I can't see any reason why Katatonia wouldn't be able to do as good in the US as a band like Porcupine Tree, for example. As it was said before, it has a lot to do with promotion. If someone would just make an attempt to get Katatonia regular airplay on some known US radio-stations like PT did, then I think that their popularity would surely skyrocket, to use a cliched term. In regards to Porcupine Tree, their song "Shallow" was played a lot on various rock radio stations all over the Chicagoland/Milwaukee area, and I think that helped them, because their Oct. 11 Chicago show was completely sold out. And because of this they had to schedule ANOTHER Chicago date the next night so people would be able to see them (they were supposed to have an off-night). Radio play can definitely help.
I've been playing Katatonia for a lot of people lately and everyone seems to love them. I don't think it has anything to do with how depressing the band is, which some people have commented on, it's just that nobody has any idea who they are! There is a "market" for ALL types of bands, whether they are more upbeat or totally depressing. Look at some of these metal/metalcore bands that are really popular in the US now, they aren't exactly the most uplifting sounding bands to listen to and yet they are really popular. Tool, for example, is a pretty downbeat band and they are one of the most popular bands in the United States! There are also a lot of crappy downbeat emo bands in the US that are popular too (I feel ashamed talking about Katatonia and mentioning emo, lol. They have more emotion than any emo band ever will). So I don't think that Katatonia's sound has anything to do with their lack of popularity in the US.
Or maybe Katatonia doesn't even want to be all that popular, which is another thought. But I don't think that claim has a lot of value because they seem like they are a very hardworking band, and I know if I put as much effort into my songwriting as Katatonia does then I would want to get my music out to as many people as I possibly can!
There are lots of ways to help the band. Play their music for people you are hanging around with. If someone asks you what your favorite bands are, mention Katatonia and ask the person if they would like to hear them. If you have something like MySpace, add the band as one of your friends and keep them on your Top8 friends list (like I do) and add one of their songs to your profile. Send their music to some of your local radio stations and ask them to play it or.... hell, you could even get you and a bunch of your friends to continually call in and request Katatonia, even if they don't have any of their songs. If a radio station gets enough requests for a particular band then eventually someone will get curious and check the band out. Word of mouth is a great tool and can definitely help a band out.
Maybe Katatonia will never be chart-breakers, who knows. But I'm sure that they can gain the popularity of bands like Lacuna Coil, HIM, Porcupine Tree, and Opeth in the US.... if only they had better promotion!