For years, Subway ruled fast food.

But the sandwich's chain future is looking increasingly bleak.

Subway's US sales last year fell by 3%, the most of any of the top 25 fast-food chains, Drew Harwell reports at The Washington Post.

Subway also fell two spots to become the third-most-popular fast-food restaurant for the first time in seven years.

The fast-food chain's problems can be traced back to two mistakes, Harwell reports.

1. Not keeping up with the competition

With its vegetables and lower calorie counts, Subway arguably invented the idea of "fresh" fast food two decades ago.

But while Subway stayed the same, better competitors got into the space.

Chipotle offers food that is raised without fillers or antibiotics and is prepared fresh in stores. Firehouse Subs and Potbelly offer elevated ingredients and side dishes such as gourmet kettle chips and potato salad.

Americans who once praised Subway's low-fat offerings are now concerned the chain's lunch meats and sauces are overly processed with fillers and additives.

"What Americans see as healthy has evolved," Harwell writes. "Subway hasn't."

2. Focusing only on restaurant growth

As American tastes evolved, Subway food remained largely the same.

Subway opened more stores without changing the menu. Today it has more than 42,000 locations worldwide, making it bigger than McDonald's.

The expansion plan is backfiring, according to The Post.

"More people have money to spend, and they're choosing to spend a little bit more on better concepts where they get a better product," said Darren Tristano, executive vice president of industry research at the data firm Technomic. "Subway's strategy has only been to open more stores, and ultimately those stores just cannibalize each other."

In other words, Subway is so ubiquitous that customers leave one restaurant to go to a closer one.