I was surprised by the sparse attendance as well. I admittedly didn't watch Sunday night, but I was also surprised at how sparsely attended their winner's bracket games were in addition. Really embarrassingly poor support by Stanford fans and for that matter, Cal State Fullerton fans, too.
West coast teams are notorious for no home crowd loyalties or attendance. 500-600 per game for the Stanford types. As good as southern California collegiate baseball is, it really does suck when it comes to attendance.
Don't know what it is unless it's part campus "academics" related or commuter status or perhaps even no home base willing to fight the traffic to drive in for "just a baseball game."