Is Southend lacking enough slots, parking space or gates to handle Carlisle ? My understanding is that at the moment SEN generally is not particularly capacity constrained, so SEN should grab whatever routes it can get, no matter how much better the route might perform at alternate London-area airports. I imagine Carlisle-Heathrow would see more pax than Carlisle-Southend, but this theoretical option doesn't exist in the real world
That said, if the flying is not profitable for Nyxair / Stobart, then there is not a lot of point in flying the route