Answer:
resource immobility
Explanation:
"Resource Immobility" refers to the ability of a company to enjoy a comparative advantage of its resources because other companies cannot easily copy their strategies and resources.
So, in the situation above, the Southwest Airlines' (SWA) offerings (strategy) are imperfectly imitable by Continental and Delta. This makes the offerings by SWA valuable to the company. This has given the company a sustained competitive advantage over several decades since the other companies have a hard time copying it.
In this kind of scenario, the strategies and resources that are hard to imitate are often rare and costs a lot to organize.