Burger King touched off the plant-based-meat revolution in the restaurant world when it launched its Impossible Whopper in August. It was followed by Dunkin' with its Beyond Sausage breakfast sandwich and White Castle with Impossible Sliders.

Subway introduced meatless meatballs, Little Caesars started delivering Impossible Sausage pizzas, KFC added Beyond Fried Chicken, and Hooters announced Unreal Wings. Taco Bell, Qdoba, Panera — the list of fast-food and quick-serve restaurants rolling out plant-based options was encyclopedic.

With one notable holdout.

McDonald's, the world's biggest food-service retailer, sat mostly on the sidelines. In September, it debuted the limited-time-only P.L.T. — that's plant, lettuce and tomato — at 28 restaurants in southwestern Ontario. Around the same time, it sent up a tiny trial balloon in Tampere, Finland, offering something it called the McVegan. On Tuesday, it expanded the experiment, offering the P.L.T., with its pea-based Beyond Meat patty, in a total of 52 restaurants across the Canadian province.

But still, food-industry experts were scratching their heads: With more than 38,000 locations in more than 100 countries, what explained McDonald's relative conservatism in the face of the past decade's biggest food trend?

Last week, Reuters reported that talks had broken down between Impossible Foods — Beyond Meat's biggest competitor — and McDonald's, with Impossible Chief Executive Pat Brown quoted as saying that "it would be stupid for us to be vying for them right now."

Rachel Konrad, chief communications officer for Impossible Foods, said that the Reuters report misconstrued Brown's comments.

"We never abandoned talks or pulled the plug," Konrad said. "The reality is that Impossible Foods wants to eliminate animal agriculture. That requires us to provide plant-based meat everywhere that animal-derived meat is currently sold, including the world's largest sellers of animal foods. We would never reject any prospective customer."

Nonetheless, industry analysts nodded. This was confirmation of what they suspected: Supply-side problems were keeping McDonald's from jumping into the fray. Even the two plant-based industry giants, Impossible and Beyond, do not yet have the capacity to supply all McDonald's franchisees.

Bob Goldin, co-founder of the food-industry consultancy Pentallect, said the P.L.T. trial is in keeping with the longtime modus operandi at McDonald's — a trial with a limited-time offer, a new menu item workshopped in a foreign market, a regional rollout to test the waters.

"Historically, and prudently, McDonald's is cautious. They are not first to market," Goldin said. "Some of that is philosophy, and some of that is the scale of the operation. To add a new flavor of beverage is a massive endeavor for the supply chain."

Goldin said McDonald's has not had any demonstrable successes with new products in a long time.

"When was the last time they innovated? They haven't had a real breakthrough in a while," Goldin said.