Everybody needs to eat, in order to produce the energy required to live, but historically there have been some significant differences about the process of eating.
For example, early in the history of eating, some people raised objections to the eating of animals. The reasons for their opposition were varied. Some held religious beliefs that led them to the conclusion that eating animals was simply morally wrong. Others had theories of rights and justice that led them to the same conclusion. Others simply felt within themselves that they weren't comfortable with eating animals. But all were principally motivated by the belief that there was something 'wrong' with eating animals. So they decided to set up their own approach consumption of food, dubbed ethical eating, which avoided including animals in their diet.
This did not go without challenge. Some felt that ethical eating advocates were trying to 'politicise' food consumption. Some went further and argued that the advocacy of ethical eating fundamentally subverted the function of food consumption and might lead to to a poor diet and, by extension, poor health. These people opined that ethical eating might even therefore be illegal. However most were willing to tolerate this new group, provided they kept their unorthodox practices to themselves.
As a result the ethical eating movement grew, to the extent that it began to create a demand for services. Ethical eating service providers began to appear, and this became a small but significant market niche. At the same time researchers began to look at the claim that ethical eating would lead to a poor diet and poor health. Surprisingly they found that there was little basis for such a claim, provided that the food was sensibly prepared, and there was even some evidence that not eating animals might lead to a better diet, and better health.
The ethical eating movement began to become more ambitious. Emboldened by the findings that there might be some health benefits to their approach, some advocates began to call for it to become mainstream. Ethical eating shouldn't be restricted to specialist restaurants and shops, it should be available as an option everywhere. Increasingly they argued ethical eating not in terms of the rightness of their approach, but of the benefits to be gained by adopting it. You don't need to believe that eating animals is wrong, they argued, to understand that eating them can be bad for you. Gradually ethical eating products and services began to appear as options in mainstream restaurants and shops.
But ethical eating advocates recognised that they faced a problem of language. The term 'ethical eating' gave the clear impression that such an approach to food consumption was predominantly a values-driven initiative. Various alternatives were adopted, with 'socially responsible eating' popular for some time, though ultimately even this was narrowed down to just 'responsible eating'. Under this label no obvious value judgments were implied, and who disagrees with a responsible diet? More and more restaurants and shops introduced 'responsible eating' options, and increasingly they signed up to industry initiatives, aware that customers may not even enter their establishments unless they were signed up to them (though whether they were actually convinced of the benefits is another question).
At the same time analysis of responsible vs traditional eating became more complex. It turned out that it wasn't quite true to say that 'responsible eating' in general was better for your diet. Rather it was the case that eating some things and avoiding others was probably a better approach. In addition, simply paying a bit more attention to the food preparation process was generally a good idea. As a result mainstream eating adopted minor elements of responsible eating, like not eating too much meat too often, rather than simply avoiding eating animals full stop. Nonetheless responsible eating advocates continue to push their case, and hope that one day they can drop the 'RE' label altogether because it will be incorporated into the mainstream, albeit with some significant compromises with their original ideals.
Now, of course, no-one really argues the case for RE in terms of the rightness of not eating animals, which frankly feels rather preachy and old-fashioned, and even a little bit embarrassing. And in any case where's the need when you can make a case for eating a bit less meat in terms of long-term healthy diet?