Smokers have the most opportunities to litter.

I imagine few people create garbage multiple times a day while outside. The people who do only litter if they can’t be bothered to find a bin, so they probably rarely visit that particular place. So smokers, teenagers and truckers just have more opportunities to litter, leaving butts, wrappers and cans.

And once you’re used to littering when there’s no bin, you do it when there are bins as well.