I recently watched the 1968 Tigers v. Cardinals World Series.
Men in the stands wore white shirts and ties. The women wore dresses and long sleeve gloves.
It is called pride in oneself and country.
I think a man can be proud of both himself and his country without sweating to death in a suit at a summer baseball game.
Also, most ordinary people weren't wealthy enough to own a lot of suits, so they'd have to keep wearing the same threadbare ones to places and venues for which they were ill-suited. Wooden bleachers, for instance.
Must smarter -- and sharper -- to save the suits for dressy occasions more suited to the durability and cut of the fabric.