Because (I believe) the legislation has made it illegal for companies to take bets from US citizens.
888 mentioned it in their conference call with investors, and also highlighted it in one of their press releases:
"These provisions state that, from the date the legislation becomes effective, the receipt of funds by electronic funds transfer and other prescribed methods in connection with internet gaming that is illegal under U.S. state or federal law will constitute a federal offence by the relevant operator."
Press release taken from here:
http://miranda.hemscott.com/servlet/HsPu...nsform=newsitem
While others (such as Poker Stars) are happy to take the risk, publicly-listed companies cannot afford to do so. They would find it more or less impossible to get people (and most importantly institutions) to invest in their company if they were breaking the law.