People don't ask for the new NFL game, they ask for the new Madden game. The Madden brand has built up loyalty among gamers, and the only way a NFL 2K game could do anywhere near as good in sales would be to do $20 again.
As for the box, look at it and you will see that "NFL" is very small. "MADDEN" is much larger font size, even the EA Sports logo at the year number are much larger than the NFL logo.
Another NFL game might be OK, but Madden has built up a fanbase and even with competition the Madden series would sell better (it's one reason the NFL gave EA the exclusive license).