It's tough, physical, football in the Championship. While the Premier has become more European in the style and tactics some teams play, the Championship is closer to the way English football had been played for years. That appeals to a lot of people. So does the competitiveness, it's not like the Premier where the top four is pretty predictable, it's wide open at the top and with the play-offs, half the division has a chance of being promoted.
There is also no Premiership football throughout Yorkshire except for Hull, and Derbyshire and Nottinghamshire or in East Anglia. There are also some big famous well supported clubs that had played at the top level for years that are now in the Championship. Derby County is drawing close to 30,000 a game, Notts Forest, the two Sheffield Clubs and Ipswich and Norwich are getting around 24,000 a game.
There are a number of historic local derbies as well in the Championship and this increases attendance. Sheffield Wednesday and United, Norwich and Ipswich, Derby and Forest, Cardiff and Swansea, Birmingham and Wolves, there is five London clubs, Southampton and Plymouth and even Barnsley and Doncaster against the two Sheffield clubs.
There is a lot going in the Championship that brings in the crowds and ticket prices could be a factor, I know some clubs have special deals that are bringing in the crowds. No surprise that it is the fourth best supported league in Europe.