Why was Elia Kazan's testimony to the House Un-American Activities Committee (HUAC) self-serving?

In the early 1950's Cold War America, the US Congress convened committees to ferret out communists, especially in the entertainment industry. Those called to testify could be arrested and imprisoned if they admitted to being Communist Party members at any time or for refusing to say if they or friends or colleagues had been.
The director Elia Kazan, formerly part of the avant-garde Group Theatre was called. Along with admitting former Party membership, he gave the names of eight Group colleagues who he said were members too. He also published a newspaper ad defending his patriotism and urging others to come forward.
Two years later (1955) his film On the Waterfront won eight Oscars, including his own for best director.
Critic Wendy Smith, in a book on the Group Theatre, wrote that his actions were self-righteous and self-serving. Many people refused to name names, lost their jobs, and went to jail; sometimes they were hired for low wages through using pseudonyms or were published by other writers called "fronts." Dalton Trumbo, for example, served almost a year in jail. He wrote Roman Holiday under a pseudonym, and when his screenplay won the Oscar, he could not accept it. Kazan was thought to be not so much patriotic as afraid of the very real consequences of refusing to talk.
https://theamericanscholar.org/the-director-who-named-names/