[Closed] why are road bike clipless pedals "single sided"?

Bought my first road bike a few months ago, and not wanting to spend loads on it, I used a spare pair of crank brothers acid clipless pedals that I had hanging around from my mountain bike. These have worked fine, but I thought that maybe they're super heavy compared to proper road pedals, so maybe I should look to get some proper road bike clipless pedals.

Looking around at proper road bike pedals, I noticed that they all seem to be 'single sided' ie, the pedals have to be the right way up to clip into them? why is this? seems a backwards step to my mountain biker brain. There must be a good reason for it. The pedals themselves dont seem significantly lighter, so it cant be a weight saving measure.