Look around the stadium and you can see how much advertising space hasn't been taken up. Prime pitch perimeter advertising real estate is taken up with non revenue earning slogans such as 'The future's bright, it's blue and white'

Inside the gents in the East Stand, all of the advertising boards are empty and the club haven't even bothered to put any club related marketing in there for several games.
Of course businesses won't be willing to pay through the nose for advertising when the team is doing so poorly but they still get the opportunity to put their product/service in front of more than 10,000 people and even more for Sky/red button matches. So why hasn't that space been sold, albeit for a lower fee?