My experience, at least in Glasgow, is that often the criminals are known to the owner of the small shop the crime was committed outside, because they're often small local shops serving small local communities… and they'll just drag their feet on handing any evidence over the police, and sometimes "forget" they were meant to and just delete it as part of their ongoing fortnightly (or monthly) cycle. Showing this was deliberate and not negligence is hard.

They don't care about providing evidence to the police, because their customers might stop going if they're going to be ratted out on, they only care about protecting their own shop.