It's to make more money, pure and simple. It's good that cricket is so popular that the final day at the Oval in 2005 was full (remember full grounds on the final day were previously pretty much unheard of). It's shitty that the authorities have reacted by doubling the price and making you buy six months in advance.
I used to go to the final day at the Oval pretty much every year. Not this year though.
ETA: If you look at the size of Australia's grounds, they dwarf those here. And everyone can get in to watch. If they can do it, so can England.