Its not just that they didn't want to give SISU any money, you also have to look at the city... In Coventry alone we've the speedway team thats winning aswell as the ice hockey team. If you're given a choice between watching pathetic football and giving money to faceless owners for pretty much no thanks/nothing returned (i'm looking at you chelski in the cup), or going to somewhere else/not spending it, most people will always pick the latter.
Whenever we have had good crowds up there, the players have bottled it. Most likely due to the pressure because its never been expected of their abilities.
When we first moved to the arena we had decent crowds, not achievement breaking but fairly decent. Since then we haven't ever really invested in the team and bar maybe one or two stand out players who were in the twilight of their playing time (Dennis Wise), we've not had much to cheer about.. So as someone said, to still be getting 15K in person is a pretty good thing.