Budgets! Budgets! Budgets! If Merlin can save a penny or two they will do it. This isn't new, we all already know this
Look at Saw: The Ride, the actors that were supposed to be in The Rack in the indoor queue were supposed to only be there during peak times (Holidays, Special Events etc), but they couldn't even keep that up.
There's also been countless times I've been through Saw Alive and there have been a minimal amount on actors
Even during Fright Nights, I once walked through The Asylum and counted six actors in the whole attraction, many running in front of me so they could perform multiple scenes.