Why did they have wrestlemania 22 at the Allstate arena?

I don't get it how come they had a wrestlemania at a venue like this i mean i know it's a big venue and all but now that they have started to have mania in huge stadiums every year why did they have it at this venue they only had 17159 for this mania it was a a **** attendance and now they get attendances of like 75,000 i don't get the wwe sometimes and also they have normal ppv's at this venue lodes and monday night raws so why would you want to have the grandest stage of them all at a venue that they use a considerable amount of times a year?