Why would a researcher use a secondary source instead of a primary source when analyzing a historical event

Either a primary source could be unavailable or a primary source could be extremely biased. For example, if a researcher was looking at King George, they might not want to use the diary of someone who despised King George as a big part of their research.

high entry costs prevent new producers from entering the market.

explanation:

an oligopoly is a term used to clarify the structure of a particular market, industry, or organization. a market is considered oligopolistic or amazingly focused when it is shared between a few companies.