Exactly, it doesn't touch enough students to receive the interest it used to. Part of the problem is the general sterilization of the game atmosphere which accompanied the move to Division I. At UCD events I've attended in the current era I remember being inundated with "this is how you must behave" messages. We all know right from wrong (except for some unnamed college students, who are still trying to differentiate left from right) so this is a little insulting, and it lends itself to a stuffy atmosphere. People know the rules at these games. As Homer Simpson put it "Just because I don't care doesn't mean I don't understand." And not caring is the reason for the poor behavior much more often than not knowing the rules.
So what does a stuffy atmosphere have to do with less interest ? Rivalries are often strongest when the two schools and students really dislike each other (or they at least say they do, but leave it at the game) When I was a student every major event (football + men's basketball, and baseball but only when a certain team was visiting) had some features where the students were encouraged to hate on the opposing school. I remember a football game in which they lynched a dummy in a Cal Poly uniform from a Toomey Field goalpost. Now that was pretty extreme but it encaptured the whole "The other team is the enemy and so is anyone who likes them," attitude that made the game more intense and motivated people to really get into it.
Now, you have more of a restrictive atmosphere where you're told what to do and be nice at penalty of cancellation. Is this better or worse ? Depends on who you are and where you are in life. I graduated a long time ago, and as an older alum my days of heckling the other team and getting all wound up during the game are far behind me, so I'm fine with "Everybody be nice" now. In reality everybody should be nice, but it's a bit naive I think (and I'm referring only people in general, not you) to wonder where all the super-passionate fans went.
And it's how society is going in general too I think. As you have more and more people who are engaging in cancel culture and trying to dictate how people should act and what they should say the closer American society creeps toward resembling Nazi Germany or the Chinese Communist Party. I'm not equating a "be nice" message at a football game with the Nazis by any means, just that the more restrictive things get the stronger the comparisons get in today's society.
But once a year we get a portal into the past where people show up in droves to hate and be obnoxious and that is the Causeway Classic. They really should change the name to ""The Nasty Bowl" because the unruly crowd generally dwarfs the hosting university's ability to regulate it, and therefore nastiness is often left unchecked. Or maybe call it the "Toilet Bowl" when it's held at Hornet Stadium ?
Now could we bottle a tiny bit of that nasty football rivalry and spread it to other sports to interest more people ? I don't really think so. If you've been to say a Causeway baseball game at Sac State or Davis it doesn't get very heated-the fans mostly behave. You might have a few people up on the Sac State parking garage yelling insults but I get the impression that this is because they live in their cars and they are just projecting their frustration with their lack of personal life success onto the Aggies.
Take out the football rivalry and the two universities are really just doing their own thing and their identities are not really intertwined.
Now, expanded intramural competition would probably be a lot of fun for the participants , but I don't see it generating much student enthusiasm beyond the events in which they're personally invested.