And the biggest metal band is Metallica (sine 30 years!). Iron Maiden is on the same level as Ozzy Osbourne/Black Sabbath - sometimes less or more but way behind the Metallica and even Rammstein. These two bands' fan base is still growing with hosts of young fans - we can't say the same about Maiden, particulary in US.
Discalimer, I'm not an American, and have never been to the USA.
But USA appears prominently in tv coverage and music charts etc. It's such a big economic market that it heavily influences global sales etc.
My perception is that USA is quite different to other "Western" markets.
USA have a massive patriotic and nationalist thing going on. So a USA band such as Five Finger Death Punch will out chart Iron Maiden in the US, but not in the other global markets. A degree of nationalism occurs in other Western countries too, but no way near to the degree of what happens in USA.
My theory is that since USA originated thrash, then there is a strong bias in USA towards thrash and the heavier more extreme styles rather than NWOBHM.
But when I look to see how "popular" a band is, I kinda ignore the USA market, because I want to gain a less patriotically influenced position.
I think, abroad, Metallica have lost a lot of their appeal, although they are still very popular. But at least abroad FFDP don't out chart IM.