Because many businesses have a particular aesthetic and if one person can post a poster (about a crime or anything else), others will want to do the same. For that reason, some businesses have a bulletin board inside the business (if it's large enough). If it's a food business, seating people at a window (so that they get a view outside) is obscured by anxiety-producing notices of crime.
Further, who polices the posters? What if the posters veer into suggesting a particular sort of person is being looked for - because the family thinks that's right? I can think of more reasons.
Target stores where I live used to have a community bulletin board, many years ago, but it became impossible to police/vet and some of it was revenge-oriented relationship stuff (people are weird).
If the police want to do it, they would likely use wording such as what you suggest - but police often do not want casual tips. People with real information contact the police as a matter of course and are persistent. They already have what, 1200 "leads" from phone calls? All of which will likely be vetted - creating a massive workload for what is likely to be a small payoff.
At any rate, I doubt there's a single person in that region who doesn't know about this crime and everyone knows how to call a hotline.