Three factors fodr those huge City Championship games back then:
1. Prep football was a neighborhood thing and was very popular. (Note the Public League was very strong).
2. It was the only championship game people could afford.
3. Most importantly, employees of the Park District (and there were many, many thousands -- both regular and seasonal) had to buy ten tickets each .. and distribute them to people likely to attend. Check out the book, "Soldier Field - a Stadium and it's City" by Liam T. A. Ford. Many libraries have it (I know it's available at the Cook Library in Libertyvillle.) It covers all the great events held there, and explains the huge attendance marks for prep football.