When the Unlawful Internet Gambling Enforcement Act passed in 2006, through the US Congress Party Poker and 888 Poker left the market, to avoid any problems with the law.
Full Tilt Poker and Pokerstars however still accepted US players until 2011. This is how they managed to be the top 2 poker sites, because up until then Party Poker was the most popular site.
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At one point Party Poker had over 50 percent market share, according to Norbert Teufelberger, CEO of Bwin.Party.
The regulations in the US are still not sorted out, however this can happen fast. Party Poker and 888 Poker both are great websites, and US players could finally join some trustworthy poker sites.
Now that both Bwin.Party and 888 Poker have earned the trust of the congress, a reentering in the US market could happen in the near future.