Noise, belts are quieter than chains.
Plus they're probably cheaper and easier to route in different configurations around multiple pulleys. They're also lighter, so they wouldn't hinder acceleration as much, I guess.
guess I'm willing to sacrifice a bit of quiet and performance to not have to worry about a guaranteed $300+ repair down the line. Maybe it's just frame of mind and I should think of timing belts the same way I think of tires or breaks.