Essentially, as Frozenbyte has said, the delay can be attributed to stringent testing for a console release, combined with a long wait for approval of every test -- with just a dash of Sony marketing strategy. Frozenbyte's Joel Kinnunen has described the issue in a lengthy forum post.
"The reason why the US version is so late is a bit of a similar situation as the overall delay," Kinnunen wrote. "We had the US version done around the same time as the European version and I think the publisher submitted it swiftly after hearing the European version was 'ok.'" After that delay, Kinnunen believes, it was Sony's desire to "give the game a good window with regards to other PSN titles/content" that kept it off the US PlayStation Store until today.