Depends on the price of fuel, cost of landing fees, crew wages and numerous other factors which airline planning departments weigh up when trying to decide if a route will be profitable or not. The difference between being in the black or in the red is often very slender with thin profit margins. Some airlines have advantages such as access to cheap labour or a favourable location. A fleet mix which may have been ideal 10 years ago might be unsuitable today.
It's a question of getting the numbers right which is notoriously difficult to do consistently.