While the official capacity was recorded as 56,864 that only relates to tickets sold and not supporters through the turnstiles.
Obviously the lack of ticket trader is a factor but you have to ask why so many season tickets are missing games so early in the season. During August you would expect a number of families to be on holiday but the school holiday season has passed.
I know of a number of band 5 season ticket holders who paid just £289 for this season who can now sit where they want. With a completely open stadium and no checks on entering any block, they can pick and choose which seat they can sit in during a game with some choosing empty band 1 £899 seats and even a few £1,100 1966 seats.