It's because roundabouts require drivers to engage their brains and use mental energy to navigate, which is a bigger sin to cars-first thinkers than asking them to wait at a red light. Carbrain in the US is first and foremost about mental conservatism.
I have no idea why car-brained people like the Mayor want traffic signals instead of roundabouts. Do they just like waiting at red traffic signals? Or do they think that roundabouts are vaguely French?