More folks watch the NBA than ever before, so I don't know where you're getting this from. I don't know how many times it needs to be reiterated that cable is dying and it has affected the TV ratings of almost every single medium of entertainment. Folks don't consume the NBA like they used to (never mind the fact that ratings have absolutely no bearing on how it's viewed outside of this country), whereas the NFL is still widely watched by traditional methods, largely because of the older demographic(s) and because folks are typically at home on Sundays and they can plan an event around it.
As if it needs to be continuously stated in here, the TV ratings for the Superbowl are gonna be sky high, probably set recent records, but it won't have anything to do with football.
Go figure.