Why the B list cast?

Probably to keep the budget smaller. It still cost $100M I think.

Because we're sick of the A list cast.

LOL.

You really like to see those movie stars, don't you?

Ensemble films are better with B list casts as the actors are not detracting from the story. Also an A list start might be able to make the film about them instead of the historical event.

For example, a movie about Billy Fiske was planned with Tom Cruise. Had it been made, we might have been surprised to learn that Tom Cruise won the Battle of Britain by himself. :)

If the actors can act I prefer to see a movie like this without A-listers. When you have an A-lister you know that they on't get killed until at most the last 5 minutes of the movie so there is no tension in anything they do. When you have b-listers you have no clue who will live or die in the movie and that makes it more engaging.

Can work with a good script. Dunkirk and 1917 were successes with a relatively unknown cast.