It seems like operating an airline is such a capital intensive industry that once you've established yourself you can 'ignore' certain norms of competition and innovation.
Commercial air travel doesn't seem to have undergone much innovation since the Concorde failed. Domestic flights don't seem much faster or more comfortable than 10 years ago, and service has arguably gotten worse as usage has continued to increase. Airline travel is still so useful that many customers will simply take what is offered because in many cases travel by other means is not possible.
It does not seem like a monopoly because there are many competitors, but I don't know how to describe what I perceive in market terms.