When the vehicle you're about to collide with is moving slowly, there are good radar returns, so you can use both vision and radar to make the decison.

When the vehicle is stationary, the radar data has to be heavily filtered because radar has poor angular resolution and there are a lot of other stationary objects, many that you don't want to stop for at similar angles, like overhead road signs. That means you're relying purely on vision, which alone has a lot of false positives and negatives, so detection thresholds need to be set much lower.

Doesn't mean it won't work. Just means it's less likely to work, especially in the dark and the rain, where radar would normally be a big part of the signal.