In 2006 the religious right & existing casinos partnered together on a law which made online poker effectively illegal in the US. Even still, the bill only passed because it was tacked on to unrelated "must-pass" legislation at the last minute in the Senate. https://en.m.wikipedia.org/wiki/Unlawful_Internet_Gambling_E...
Adding context to the original question (from Wikipedia):
"According to a study published in May 2015, players from the USA are banned in more than 72% of the world's online casinos. Having considered multiple factors, such as ISP blocks of online casinos and restrictions for casino financial transactions, the report concluded that as of 2015, the USA is the worst country for online gambling of the 225 studied[1]."
