... the US Department of Justice decides to reach beyond its own borders to crack down on internet gaming. After sneaking anti-gaming law into "port security" legislation last year, they're now getting serious about enforcing it. Companies (most of them offshore) are losing a ton of their value in the market and many big names have pulled out of the US.
So poker (where a player can control the outcome to some degree) is out, while horse racing and lotteries are still in. Makes perfect sense.