I think there's a lot of truth in that; as you say we've got nothing to 'fight' for any more. Yesterday was just a bog-standard, humdrum league game against opponents who are a good side but not one of the top clubs. And there was nothing really riding on the result as we're not even halfway through the season so there's plenty more matches in which to pickup/drop points. Consequently there was no sense of anticipation or expectation, so basically what was there to get excited about?
Plus of course the Brighton fans that attend matches (along with fans of other PL clubs) are nearly all nice comfortable middle class people with nice comfortable middle class lifestyles who can afford to spend £25+ on a match ticket. I'm no sociologist, but I think these people tend to view it more as ninety minutes entertainment that elicits polite applause as opposed to a tribal encounter that demands passionate, unconditional support..
Nailed it - would just add that we've got a large cohort (obviously) that can afford to spend £25 on a ticket and not go..................