Just my educated guesses as a layman, but I agree with what others are saying in that a center channel would lead to more flooding in the streets. Side drains won’t flood streets or homes (unless its an actual flood) as both are elevated above said drains. Side drains also give 2x the holding capacity which further helps reduce water pooling on the road.

Additionally, having it in the center of the road means any maintenance done on said drains is going to block both lanes of travel, and even if it doesnt, would put workers in a very dangerous working position.