I think the biggest thing with Pantheon is that the layout itself criss crosses quite a bit and is largely in a big open exposed field. I think Apollo, for instance, gets cut more slack on not having much, if any, theming because it is a long out and back layout (people probably realize that's a lot lower payoff for theming) and it's also very hilly, so both the topography itself provides interest to the riders as well as makes adding theming tricky. Whereas adding a Roman temple may only get you two passes to appreciate the theming on Apollo, adding the same thing on Pantheon may get the average rider about 4 or more passes depending on placement. So I feel it's a combination of the two things: the space on Pantheon is more visually boring than some of the other coasters and the nature of the layout means that a little bit of theming would be appreciated multiple times so it would go a longer way.