It's an open market, so countries can't invent subjective regulation to block competing identical eu products from coming on their market. I've wondered how this price difference is sustained and I have no clear answer either.

The only thing I could think off was that Belgians (I'm Belgian) and French people are very snobistic + chauvinistic about what we eat: origin is prominently displayed on meat and dairy, so it's probable that enough consumers are willing to pay more for perceived better quality, so that the supermarket chains keep buying local. To sustain the believe of the population in the superior quality of our local products, problems that could threaten this believe must be taken seriously.

In the Netherlands this is different: the dutch only care about the price, it's no coincidence that the dutch invented the water chicken.

A few years ago we had our latest scandal in a Belgian slaughterhouse (there's been many over the years): it had a bad reputation, some people went undercover and made footage (it was no where near as bad as a German slaughterhouse, but still ..., could have been a lot better), the footage was published, public outrage followed, inspections followed, the slaughterhouse was temporally shut down, business was lost, expansion plans scrapped, responsibles were prosecuted and ultimately received a slap on the wrist fine.

So slaughterhouses in Belgium do have problems, but scandals have consequences that the slaughterhouse management really wants to avoid, and that's why I believe that my chauvinistic belief in the superiority of Belgian/french products is warranted. 😁

Edit: the slaughterhouse scandal that I mentioned was about ill treatment of animals, not for hygienic reasons. About 8 years ago there was a case of a slaughterhouse failing inspections repeatedly over a 2 year period and they were shut down after failing to make improvements. The inspection takes hygienic and qc issues very seriously, but seems to have higher tolerance for animal cruelty than the general population.