I think it's down to the marketing also, barely anyone knows about Face It Alone! I wonder how busy it is on Saturday's/Half term dates, as people don't have work/college next day.
It would be interesting to see them implement the Face It Alone experience in it's own maze that's open during the day (maybe 6PM-10PM) and charge £10/£15 each for it. That would give people more of a chance to experience it, as it's a long day at Thorpe to then stay later after 10PM.