Mike Oeming wrote the final arc of Thor's series. As far as I know, sales on Thor were quite healthy--healthier, in fact, than Captain America and Iron Man, both of which were only canceled to be revived in new series, with new #1 issues that would generate interest, a short time later. Based on Joe Q's comments and the fact that Thor's sales weren't low, the only reason I can think of that his series was canceled is because no one really knew what to do with him. Joe Q mentioned on several occasions that Thor didn't "fit" in the Marvel universe (which we all know is utter trash, since he fit just fine for 30 years).
But I'm not bitter.