Partially when the law was changed to allow up to £300 worth of goods to be removed without payment, unless violence was involved, before they get involved.
That and people feeling that shoplifting was a victimless crime. I've spent four evenings in hospital after being attacked at work.
Technically then law wasn't change. The dibble have dwindling resources and increasing workloads so have little choice but to prioritise, and some (not all) use a value based calculation.
My old farce do it on an evidende based assessment. Does the staff member know the suspects name? Is their CCTV that clearly shows their face, selection of goods and concealment of goods, and then passing the point of payment? If the answer is "no" its an evidential dead end and pointless sending a bobby that could be used elsewhere, such as hassling people for airing a legal opinion on X.
People get confused besause theft below falue £200 and theft above have different charging codes, but the law itself is still the same for both (the theft act) and theoretically the punishment can be the same, although sentencing 'guidelines' tend to treat it on a sliding scale.