The McRib is objectively gross...which doesn't stop me from stuffing my American face-hole with the damn things every time they're available because despite being gross they are tasty.
They used to be a full time menu item but I guess the pork got too expensive so McDonald's will buy pork when it's cheap and stockpile it until they have enough to do another limited time promo with them. Been that way since somewhere in the early / mid 2000s if I remember right.
McRibs were never a full time menu item, and are seasonal at best. I suspect it more has to do with promoting customer interest during the late summer.