One possible reason: some manga has caught up to the original Japanese manga to a point where there's just one year between the original Japanese manga and the American release. Thus, to give some time between the Japanese and American version, the wait for the release of the American version sometimes becomes longer and longer. I know this because that's what going on with Negima (which has a one year gap between the Japanese and American versions, with the American version being released quarterly a la the Japanese versions).
Toon Addict for Life and "gated community" member since August, 2002.
"ph34r t3h cute ones" -Megatokyo