When running a national food chain one of the most important characteristics is making all your products taste pretty similar. Your prime customer is someone who comes in regularly and gets the thing he expects each time.

With coffee that means you want all your coffee to taste the same day after day, and year after year. Doesn't matter if your buyers bought extra beans from Brazil or Sumatra or Ethiopia because those were the years those nations had perfect weather and bumper crops so your bill of materials is down 15%, the coffee needs to be the same or close to the same every single cup.

So roasting enough that most of the flavors are coming from the roast is a really good way to ensure consistency. And because everyone's chain coffee is like that, they want their home coffee to be like that too. So dark roasts sell better and soon light/medium roasts become a niche product.