Simple, times change, as has the mix of guests at the parks. And while not "100% PC", lets basically say that Dads would stay home, or now, head to the ESPN Zone, and Mom and the Kids would spend the day at
Disneyland instead of the game.
Yes, the Super Bowl is a very popular event and has a lot of folks stay home to watch the game, have a watch party etc. But it isn't for everyone, and since many locals have Annual Passes good for that date, and then foreign visitors who also are not big NFL fans also are in town, it has become a regular Sunday in regards to crowds, and they are busy.
As for special events, it depends on a few things, there are plenty of them at Disneyland for things like the Rose Bowl game, or when the area hosted the College Football Championship a week later. (The event main location for the week was Orange County, even though the game was held in the Rose Bowl Stadium).
The first one is, does Disney have a marketing deal for the year? Who is the winning team, and where that game is played and which player got picked to say "I am going to Disneyland (or Disney World, or maybe even now Disney Parks". And then Disney also looks at which park they want to get the most attention, and have the player parade at that location. So you might get one in 2017. But in 2021, when the Southland hosts the Super Bowl in the new Rams stadium, there will be some serious impact and very likely a celebration parade on the Monday. As with many things with Disney, only time will tell.