Alda was a nobody until MASH. He only became the central character of Hawkeye and his name started with A which made him the "star".

I don't know about the book, but in the movie version Hawkeye arrives first and Trapper appears later.

Alan Alda was not an unknown. He had a few acting credits to his name. Plus, his father was Robert Alda, who had a prolific career.