Because it was, and no doubt still is, revolting. Such things are legacies of wartime hardship when meat was so scarce that the most innovative approaches were used to ensure that people got sufficient protein.
I don't know about anywhere else in the UK, but in the NE of England a very cheap product available from the pork butcher's was 'Brawn', a concoction made from those parts of a pig not suitable for display in the shop window and made with jelly. Absolutely foul stuff! As a very hungry child in the 50s I would eat anything apart from brawn, tripe and cow heels. Even now, after more than 50 years, I can still smell the tripe cooking in the pressure cooker (state of the art appliance at the time) and going out to be quietly sick.