Why not though?
The park has posted opening hours in advance so you can plan your trip to the resort in advance on a day which suits your needs and time within the park.
I'm sure the team at Alton towers would love to keep the park open late into the evening but just like any other business they have to look at it from a profit protection perspective, if there's more staff then guests in the park, which I don't think is a problem the likes of Europa would see, then how is it going to be beneficial to keep the park open late.
Besides we don't know if there is an operational constraint with the local council and noise impacting the amount of hours an attraction is allowed to be open for yearly.
Unfortunately this isn't an attraction which sells out months in advance on prebook alone. Perhaps if the park scrapped MAP and charged £220 a year for a pass then maybe we would have a different story.