Demographic change is a big part, there’s no getting away from it.
The Venky destruction of the club came at the worst possible time as the 2010s saw a big chunk of the fanbase move further away from the town. Now that isn’t an issue in itself, many clubs have seen their original patch change and fans spread further out, I’m thinking West Ham, West Brom, Bradford, Leicester etc, those clubs are still pulling in the crowds despite the fanbase generally being more disparate. However, here the owners have deliberately dismantled the club *and* the fanbase has moved, it becomes harder and harder for fans living 20, 30, 40 miles away to make the effort when the regime doesn’t.