I think somewhere the NFL had discussed switching to Saturday, but there's one reason they don't. Sunday night, they have the entire evening to themselves. Rarely is there even an afternoon game in the major leagues. And few in the minor leagues. The NHL and NBA are not giving up a Saturday night even if it is going up against the Super Bowl. And the minor leagues would face an even bigger issue. Throw in high school sports also being played Saturday night. There would be a huge pushback against vacating that slot to give it to the NFL.
I'd imagine teams, leagues, associations would be complaining to their elected representatives over it. Right now, the NFL has the night entirely to themselves, and with little pushback by anyone else. It's reached the point the other networks don't even put up much of a fight against it because when it's their turn to host, they too, do not want any competition to drag down even a fraction of the viewership.
As for the 6pm kickoff. It's because Sunday's 'primetime' for TV ratings starts at 6pm ET. Other days, primetime starts at 7pm. That's an antiquated reasoning. But it's the reason that it's still done today decades later. Also, a 6pm ET 'telecast start' of the Super Bowl means it starts at 12pm in Hawaii and the Aleutians. Giving the entire US a game kicking off after noon.