Well there's a short and a long answer and I can give you both.
The short of it is that Spinefarm has been weak US. From about 2007-2009 they had a decent US situation with Fontina/Universal doing their distro, but I am guessing Ensiferum, Swallow the Sun, etc flopped (I think the last Ensiferum did a little bit over 1,000 first week, which is terrible for a band that spent so much time in the US. I mean, they did Paganfest, direct support for Amon Amarth, a tour with Hypocrisy (although Hypocrisy bailed), and Summer Slaughter -- I think expectations were much higher. STS also did a pretty large sum of touring between those years as well over here and I don't even think the last record even had a chart debut. It's a shame really, because Spinefarm is a great label with talented people at the helm running it, but for some reason, they never were able to get success here. Even Bodom was licensed to Metal Blade for the last one, and according to rumors, even THAT release was a disappointment from a sales standpoint. Nightwish has always been licensed to Roadrunner for over here. So at this point, I can understand why this country is not a priority market for the label considering that they are extremely successful in Europe and the UK.
The long of it is that retail distributors have different release schedules and really set the times for release. Spinefarm and Swallow the Sun may want to put the record out around the time of the European release, but whoever is distributing the label now may not be able to due to its schedule, and being able to meet those two schedules at the same time is an impossible dance (especially considering that there are also publicists that need about 3 months to push the record). That's why even Nightwish for example had to get released during different periods throughout the past 2-3 months. December is typically a no-no for releases in the US, because the CD buying population decides to mainly spend its money on Christmas albums or best-of compilations since those make great gifts. Michael Buble is one such example of how successful Christmas albums can be during this period, and trying to compete with that is suicide. So naturally, Roadrunner would release the Nightwish record over here in January and not December (which is when Spinefarm and Nuclear Blast put out Nightwish). There are also other reasons too -- sometimes a band may be signed to multiple labels for different territories. Sometimes one label might have it in the contract that they get to release the record first. This is a pretty common thing in Japan, in which the label can demand to get the record released 2 weeks before everyone else at a minimum. So yeah, often times trying to sync retail schedules together is simply an impossible feat.