I don't have any specific evidence for this but my theory is that the Barnsley end has sold out in respect of the lower and middle tiers. I think they will have sold the whole half circle so it looks good on TV half the stadium red half blue. However they have likely not sold much in the upper tier. The safety issue now is if they open the upper tier to Wednesday there will be fans mixing outside and on the concourse inside and there is a risk of fans chucking stuff at each other both upwards and downwards, so I guess that is a flat no no. Whereas it would have been sensible to sell Barnsley a third of the stadium on all 3 tiers, then leave the empty seats to the end whoever has bigger demand, us of course, but they would be too sensible wouldn't it.
I have really strong belief that I'm right here, got good gut instinct, oh and my gut says 3-2 to Wednesday.