Because it is not even a remote exaggeration to say that in order to truly make the morally "correct" choices everyday, you would need to not participate in any part of society.

Telling people to feel bad about eating animal protein but to keep driving their cars that destroy the environment, shopping at stores that underpay their employees, purchasing items that are made with diminishing resources in countries that pay close to nothing to their labor force is picking an arbitrary battle in a war of existence.

Promoting making better choices will always be more effective than asking people to feel guilty over existing at all.

Source your food locally if you can, cook and eat only what you need, etc.

Because people would just feel bad about it and keep doing it. I don't care about people feeling bad about immoral behavior, I care about them not doing it in the first place.

It's shame/guilt/insecurity, whatever you want to call it. You'll get unprompted "Oh, you know, I wanted to be a vegetarian but I love eating bacon too much", or people who try to stump you with "gotchas" like "but where do you get your protein?" (remarkably, this can come from people who were born in a culture where vegetarian proteins are common!) or "I saw a thing online that says plants have feelings too so really you're just as bad as me".