But the US is a really big market. Something can be a niche everywhere else and still make a ton of sales.
Madden has cumulative sales, according to Wikipedia, of 250 million copies. The FIFA series has cumulative sales of 325 million.
I'm having a hard time finding consistent annuals sales numbers. Random Googling suggests each new FIFA year sells around 24 million, and each new Madden year around 5 or 6 million.
But Wikipedia says Madden was at cumulative 100 million in 2013, which combined with them saying it is 250 million now, gives an average of over 18 million a year.
Maybe the numbers I found on annual sales were only counting sales of whatever was the latest version? In that case, if a significant number of people who buy Madden X do not do so until after Madden X+1 comes out that could explain it. Madden X could sell 6 million to the people who want the latest version, and then another 12 million in subsequent years to people who wait until it is a bargain bin game.
Sure it can generate tons of sale, but "best economics of any game franchise in existence."...
[1] contains some numbers, Madden is probably placed around number 30 if you look at only the numbers for video games.