The reason is due to ratio of garage space to cabins. If you run out of cabins/seats, it's difficult to sell the space (impossible if the sailing is to Spain). This applies to Etretat, Connemara, BDS and CF. It's not normally a problem for travel to France as there is an allocation for foot passengers, all be it a low one. I would suspect the allocation for foot passengers has been sold.
People seem very upset about this as it obviously looks very environmentally damaging and to maximise profits. But obviously it is about profits. If a large number of cabins were occupied by low fare foot passengers, the profitability would drop substantially. Potentially enough to stop the route.