I don't think you can compare the two.
1. 9/11 was clearly terrorism, it could not have been anything else. 587 could have been caused by a number of factors.
2. Al Queda didn't need to claim responsibility, because within in days the government was blaiming them. There mission was accomplished without having to claim responsibility.
3. I believe that since 9/11 videos of Osama have shown him accepting responsibility.
