Yes, and I can tell you why (although it may sound like a lame reason).
If the audience were set up bleacher-style (a' la Tattletales), there is a strong possibility that when Johnny (or Rod/Burton/Randy) called someone to \"C'mon Down\", they could, in all their excitement, fall down the bleachers and get severly injured.
At least, with a sunken audience, the contestant is running down a ramp of sorts, and not stairs. It is enevitably (sp?) safer.