Whilst from a guest perspective its great to see them extend the hours, I totally understand why they are reluctant to do so sometimes. From a management perspective, making the call for an extension means that you are going to go way over budget for that particular day… add up all the staff that need paying an extra hour and the running costs of rides, and it wouldn't be a small amount. Not to mention forcing staff to stay later than they anticipated is not going to make you very popular, especially if they have already scheduled other commitments around their shift. Bear in mind that Alton is in the middle of nowhere so in extending someone, that member of staff may have missed their only bus home and be stuck, or be severely late for a pre arranged plan that evening.
Its great to see them finally looking out for the guests, and is frustrating when you have a 5PM close on a busy summer day, but extending the park isn't just something you can do on a whim.