Seats were cheaper at the wooden stadium over in Fidenae.
http://en.wikipedia.org/wiki/Fidenae
Stadium disaster
In 27 AD, an apparently cheaply built wooden amphitheatre, constructed by an entrepreneur named Atilius, collapsed in Fidenae resulting in by far the worst stadium disaster in history with as many as 20,000 dead and wounded out of the total audience of 50,000.[5][6]
Nice link, thanks! The original large circus in Rome used wood for the risers or bleachers, and most of its construction, and went in the fire of Nero. It was rebuilt as the Circus Maximus, with much more stone. Some of that was quarried out over the centuries for use in new construction, some of it remains visible, some of it sank into the swamp.