The steel supports are stronger than wood, and are likely going to be less expensive, so they don't have to re-engineer another entire section of wood support, and place the footers for them. The steel they used doesn't need many footers (looks like only a handful) and there's not a whole lot of steel there, so it was probably the most cost effective method (as well as the quickest way to do the job, so the ride can open on time).
Wooden coasters use steel all the time, mostly for crossover support. I vaguely recall seeing some that use steel for turns that hug the ground.