Following are the reasons due to which it becomes difficult for a new entrant to make an entry into the aviation sector:

Mergers: Many airlines merge with the other airlines to have control over airport hub in large cities which makes it tough for a new airline to make an entry. For example: Indian Airlines-Air India, Jet-Sahara, Kingfisher-Air Deccan etc.High Oil Prices: High Oil Prices means passengers of full capacity in aircraft and this is not possible in small airports as full capacity passengers can be found only in the case of bigger airports in bigger cities.CAR Restrictions: There has been high CAR restrictions on the new entrants in the airline industry due to which it becomes heavy in terms of cost for a new airline to make the entry into the airline industry.

Airport Infrastructure: Sometimes, due to lack of efficient infrastructure in airport it becomes difficult for a new airline to make an entry as every airlines requires specific runway, place for aircraft etc.

The below diagram describes how the Porter 5 Forces is relevant in this context:

(Source: Flatworldknowledge)