People need to pee way more than poop, most people using a public restroom just need to piss. This is especially true at public places that involve drinking (bars, concerts, sporting events). So having a thing you can just walk up to and piss in is more efficient than having people have to get in and out of a stall. Also poopers don't have to compete with pissers for toilet space.

Plus their easier to clean, especially when you're dealing with people who have bad aim, which also connects back to the alcohol thing.