Sheffield United get great crowds now because they've had some success in the last 20 years. A bit like Leicester/Southampton/Stoke - we had very similar if not bigger crowds in the 90s.
As football has changed, crowds have rocketed in the 21st century. The difference is, we have had the worst of it both on and off the pitch. Our crowds have declined.
We've had one decent season in the championship and are now averaging towards 20k. If we had two promotions to the Premier League like Sheff Utd have had in the last 20 years, we'd have grown our fanbase.
People always refer back to our crowds in the 90s and say 'we hardly got more than 20k in the Premier League'. Well, most similar sized clubs didn't.