Apparently it has to do with the wording of the ban, which requires the states to enforce the ban rather than having the federal government enforce it. That apparently violates the tenth amendment of the constitution.
I saw this earlier, and was curious about the details. According to the article sevenwood linked to it was, “a federal law that required states to ban gambling on the outcome of sporting events.” If that’s an accurate description it’s blatantly unconstitutional. I can’t imagine how it got passed or how it lasted twenty-six years.
There is a difference in the states having to abide by federal bans and the federal government requiring the states to take affirmative action. The later is considered Commandeering and is unconstitutional.