Car parks are not on the base due to security of a USAF airbase (post 9/11) that is lent to RIAT =
Car parks have to use surrounding fields as loaned / supplied by local farmers / Landowners (Who may charge for the use??) and happens to be on the same flood plain as the Thames =
Unpaved land that with the addition of "ONE MONTHS RAIN IN 72 HOURS" + the churning action of wheels / feet =
Quagmire!!
So thats the reason. Simple really!!
PSP.. From who and who was going to lay the amount required to support 25,000+ cars in a day??