Because all game play needs to be in one currency and the US has always been the main currency. There is talk going around that with the ban in the US the currency might be changed to the Euro which would be nice because it would mean the ABI would be higher and would lead to higher ROI
Of course it does, but you also need the choice. When the poker industry initially began you could choose between currencies and play on tables specifically for that currency. Infact ipoker network (maybe 888) was doing this up until recently where they would convert pounds into US dollars then reconvert it back into pounds once you were finished playing on a specific table
Just like the guy betting on a soccer match has the choice to play with a bookie in his currency over the net, why doesnt a poker player?