I used to see the BA internal newspaper. On several occasions there were letters from staff along the lines of
"why don't we fly to XXX? Their national carrier always seems to fly full".
To which the answer was along the lines of "we've analysed that route and we don't believe we can make money from it".
Essentially, BA don't start new routes without an expectation of profit. Amongst other things that means a good "premium" load. It's been interesting to see how they've used 787s to start new routes. For instance Austin went from a 787 to a 777 to a 747 and now A350. Actually, I say "premium" load. They also take into account freight. I don't know if it's still true but at one time it was said that the Houston route didn't need to carry any economy (World Travller) passengers. They made enogh money from the front and belly of the aircraft to make a profit.