I wouldnt say that about Xbox, they've actually sold more in USA than the Wii. The xbox has failed miserably in japan with only around 530,000 sales with around 10 mil in USA. While Wii has sold around 5 mil in japan, 8 mil in USA. Granted the Xbox was out for an extra year, still those japan sales hurt it tremendously.
But anyway, the Xbox Arcade version ($270 with 5 free arcade games) was sold out everywhere also. I guess price really matters ALOT.