I think it could be two reasons behind the lack of success of your site with Americans.
1) Like others have said, it is difficult for Americans to deposit and withdraw after the UIGEA. And perhaps that women (most bingo players are women) are still more hesitant than men to attempt to bypass the UIGEA.
2)American bingo is based on a 24-number card, plus a free square, (five rows of five numbers), and 75 balls. European Bingo is often based on a 27-number card and no free square (three rows of nine numbers) and 90 balls. Maybe you promote European Bingo Sites and Americans prefer Americans Bingo style of playing? Obviously it is only an assumption on my part.
" Money won is twice as sweet as money earned"
Eddie Felson in the movie The Color of Money