Saturday’s Major League Soccer Cup final features what would have been a dream scenario when the league launched 22 years ago.

The Portland Timbers—with their eight-year streak of home sellouts—face second-year Atlanta United, already the pre-eminent MLS club in attendance with a league record of more than 53,000 a game. The title match, which Atlanta is hosting at its own Mercedes-Benz Stadium, showcases MLS’s ability to grow large, rabid followings.

But...