This is the problem.
There are a bunch of clubs similar in stature- us, Forest, Leicester, Derby, Southampton, West Brom, Brum etc.
All of us went through similar periods of trouble...
...however we are the ONLY one who have never recovered, and the ONLY one who have had nothing to get behind- so they are streaking away. Right now if we had successive promotions we'd pull 20-25k regularly without breaking sweat, but the longer it goes without recovering the more our support is chipped away, new generations won't be there. Then you become like a Preston for example- enormous 'famous' club in the distant past but too long with nothing means their support was eroded and that takes decades to build back up.
I went to Old Trafford when there were 28,000 there; I was at Stamford Bridge the week before Chelsea played an FA Cup Final when there were under 9,000 there; I've been at Villa Park when there were 15,000 there.
Man City are the best team ever assembled in the Prem and they rarely have a full ground (check the info that was uncovered about how many people actually attend v how many tickets sold), Arsenal don't sell out, Spurs get crowds 5,000 below the capacity of their new ground at Wembley (does that mean their new ground is too big for them?)- Burnley only sold out 4 times this season and WBA never seem to sell out.
You can't just look at the raw figures, that doesn't tell you anything. Its about history, where you stand as a club, where you should be- if we had followed the same trajectory as the other clubs of our ilk, we'd be at 25k+ most weeks- no bother.