(AP Photo/Carolyn Kaster)

On Thursday, Panera became the latest fast-food chain to announce its plan to use only cage-free eggs. The commitment, which the company will carry out by 2020, is quickly becoming an industry-wide standard: McDonald's, Burger King, Dunkin' Donuts, Starbucks and many others have made similar announcements.

But cage-free eggs, which come from hens that are free to move and lay eggs in nests, are hardly the only promise fast food companies are making these days. Raising chickens without antibiotics, which Panera made a priority more than 10 years ago, is something many others are now offering or working toward achieving. Efforts to end the confinement of pigs and cattle are becoming more popular, too.

The reality is that animal welfare, broadly speaking, has become something that people care about, and companies have moved to honor. It's something you have to do, or at least have to seem to want to do, if you want to woo customers.

That is, unless you are Taco Bell, Pizza Hut, or KFC.

"When you look at the major fast food brands, Taco Bell really stands out," said Leah Garces, who is the U.S. director of Compassion in World Farming, an animal rights group. "They're the only big player in the United States that doesn't have plans to change how it sources its food."

All of the brands belonging to Yum! Brands, the parent company which owns Taco Bell, Pizza Hut, and Kentucky Fried Chicken, were given an F grade in a recent report by six organizations, including the Consumers Union and Center for Food Safety, on industry sourcing practices. What's more, it has made no clear promises to fix that.

The closest the company has come was last year, when Greg Creed, who used to served as Taco Bell's CEO but now is at the helm of Yum! Brands, told The Wall Street Journal that he would like for Taco Bell and its sister brands to switch to hormone- and antibiotic-free meat, but that it wasn't currently possible.

There hasn't been a peep about animal welfare since.

Why the wait?

Yum! Brands didn't respond to requests for comment on Thursday. But it's easy enough to guess at why the company has been slow to join the movement. There are many reasons, after all, why brands like Taco Bell might see these sorts of commitments as too risky.

"The industry is shifting toward better animal welfare rights as a corporate social responsibility, which has very much been pushed by consumers," said Darren Tristano, who is the executive vice president of a restaurant market research firm called Technomic. "The issue is that the supply chain hasn't been able to keep up."

The clearest example of this problem surfaced last year, when Chipotle realized that a major supplier wasn't meeting its animal welfare standards. The company stopped serving pork at hundreds of restaurants around the country, while it worked to replace the lost supply. It took half a year, a foreign company, and two of Chipotle's most disappointing quarters on record. Farms that raise pigs outside of gestation crates simply don't represent a large enough portion of the pork industry.

The same difficulties are dictating the terms of recent commitments to use only cage-free eggs. Starbucks and Panera are giving themselves five years to make good on their promise. McDonald's, which is the single largest buyer of eggs in the United States, is giving itself 10.

But Yum! Brands' likely has profit reasons, too, especially at a chain like Taco Bell.

More humane farming practices are less efficient, and, therefore, more costly. At chains like Chipotle and Panera, which have built their reputation by charging a little more for something customers feel good about buying, this is less of a problem—they can pass some of it on to consumers. The same goes for places like McDonald's, which have already begun their climb upscale.

But at Taco Bell, where cheap food is still the overwhelming sales pitch, every penny matters. "Taco Bell is at such a low price point, even for fast food, that their value proposition makes it really hard to switch to any sourcing that will affect price," said Tristano.

This is especially true for their breakfast menu, which was first introduced last year. The strategy has centered around novelty—there was a waffle taco, then a biscuit taco—and convenience: All of their offerings are easily eaten on the go, many with only a single hand. But its appeal is also predicated on price. Switching to cage-free eggs could compromise that, and with it, the chain's chance of elbowing out some room in the fast food breakfast space.

The simplest explanation, however, is that Taco Bell hasn't followed the industry because it doesn't have to. Its customers are young, like those of its competitors, but they are predominantly male, which, according to Technomic's 2015 food trend report, means they're less likely to care about animal welfare.  They also aren't quite as affluent as those who frequent other chains, which, Tristano points out, likely means they are more price sensitive.

"The lower you get down the price points, the more your consumer has to prefer lower prices to better animal welfare rights," he said. "So I think it's also a reflection of how Taco Bell's customers feel."

Following the leaders

Critics of fast food companies that haven't changed their practices insist that animal welfare should be a priority, not an afterthought. But getting brands like Taco Bell on board is going to require a better alignment of incentives.

One way is simply for industry leaders to continue to push the system toward a more humane equilibrium. The more companies there are that demand cage-free eggs, the more producers there will have to be to provide them. In time, the increased supply will drive down prices.

That's why McDonald's commitment to cage-free eggs was so momentous—the company uses more than 2 billion eggs in the United States each year, or roughly 5 percent of all eggs produced in the country. It's also why announcements like the one from Panera shouldn't be taken for granted.

"More and more, we recognize that we need to be part of fixing what is without question a broken food system," said Sara Burnett, who is the director of wellness and food policy at Panera. "I would hate to speak for another concept, but we make these choices, these decisions, because we want to create change."

As part of the news, the chain shared an update on the progress it has made toward achieving other animal welfare goals. As of this year, all of its pork comes from pigs raised without gestation crates, all of its poultry is raised without antibiotics, and nearly all of its beef (89 percent) is grass fed and free range.

Given the sheer size of Panera, which operates nearly 2,000 restaurants, and sold more than $2.5 billion in food last year, the shift in its sourcing will surely lead farmers, hoping to earn some of its business, to change their practices. Given the growing number of other fast food companies that are making similar commitments, the effect will likely be vast.

The silence of outliers like Yum! Brands only grows louder each time a competitor shares a new commitment to animal welfare. "Taco Bell is waiting for the prices to catch up," said Tristano. "But customers are also increasingly demanding more ethical sourcing. Young ones, in particular, who are Taco Bell's biggest audience—they see what is being promised elsewhere."

"It hasn't crystalized yet," he added. "But it's getting there."

At some point, using conventionally raised animal products will no longer make sense—not even from a business standpoint. The cost of doing nothing will exceed the price of following along, and companies that haven't joined the movement will then have no choice but to make an announcement of their own.