Poker sites do need traffic to attract players. Players love choice of tables / type of games / tournaments / not having to wait before a game starts / etc.

Despite "black friday" (when the U.S. DoJ confiscated all the dot coms from the four biggest poker sites) PokerStars is still number one. Moreover PokerStars bought Full Tilt (which died due to their ponzi scheme being discovered sooner than they had planned "thanks" to Black Friday). Just look at pokerscout.com's main page and you'll see just how big of a % of the market PokerStars+Full Tilt are representing.

Long story short: unless one of the major site starts accepting bitcoins bitcoins ain't going to grab a big share of the online poker market.