Without sounding like I'm sticking up for Merlin (I most certainly am not) I think the location of towers causes serious travel issues for staff. Considering it's a park propped up by kids, many of which who won't drive, they are totally dependent on getting public transport or getting a lift from somebody.
As we all know.....it's not an easy place to get to and I don't think the bus companies would keep running buses until 9/10pm purely for staff. It probably wouldn't be cost effective for them to keep a bus driver working until that time.
Hate Merlin as much as anyone but I think there are a lot of issues out of their hands quite frankly.