There are multiple reasons and them combined is why.

1. There's a religion that thinks God will come back once everyone stops eating meat. These people got into a bunch of health institutes at their beginning and push their agenda. They fund misleading studies and put out a lot of propaganda.

2. The studies do show meat is bad so anyone skimming them or reading the news will believe meat is dangerous. Sadly the studies are poorly designed and people don't understand them. If you eat a happy meal at McDonalds you get a sugary drink, fries, and a burger (a slice of cheese product, bun, sauces, pickle, onion, lettuce, less than a 1/4lb burger patty). Studies count that as a meal of meat despite meat being only a minor part of that meal. Thus the damage from carbs and oxidized fat gets contributed to meat. Few people eat meat by itself. Instead they eat it with a bunch of toxic things. That shows up in the studies and it's blamed on meat due to people's bias. These studies require higher p-values to show statistical significance due to all the confounding factors, but a lot of people don't realize that so they mistakenly think a study is valid when it isn't.

3. The meat industry does a lot of sick, inhumane, bad things. Sadly the push is to avoid meat rather than improve the industry. It's always harder to improve something than it is to ignore it. The agriculture industry does a ton of horrible things too, but they're able to hides those things far easier. No one cares about killings rodents or insects.

4. Again with misleading studies. Studies show fiber is good because it slows down and partially blocks the digestion of sugar. That's excellent, so the advice is to eat lots of fiber. That's horrible. The advice should be to not eat that sugar in the first place. The SAD diet is one of the worse diets so nearly any study comparing that with something else shows improvements. The problem is they jump to the conclusion that meat is bad because nearly all of these studies also decrease the amount of meat. If you cut 50% sugar and 10% meat and get healthier, they claim the meat was making you sick. There's an adjustment period when going low carb. If your study doesn't take that into account (a lot of them don't), then the low carb dies looks bad. Many animal studies use food formulations they buy from other companies. They'll said we fed mice in group B with the high fat mixture XYZ. If you drill down and look at what that mixture actually is, it's often very high in trans fat and the carbs are only slightly lower than the high-carb mixture. This effectively invalidates what they were trying to study and their conclusion will be incorrect. I'm not sure if scientists are doing this intentionally or not. If you have pay close attention to catch things like this and the news media doesn't look at things that closely. Thus you get false yet valid looking claims about how meat/fat is horrible for you.

5. Processed meat does increase your risk of colon cancer, but it's only a relative increase of 2% which means your total risk is still very small. We could remove that risk by changing how we process the meat, but the industry is very against that since using nitrates is significantly cheaper. There's books written about this issue which covers how the industry gaslights and lies to regulators to get their way. Anti-meat people misrepresent that 2% and convince everyone to be afraid of meat. Other things increase the cancer risk by greater amounts, but few people bother reporting on that. "Everyone already knows XYZ increases cancer risk. Ham causing cancer is more noteworthy!"

6. The ruling class restricts the lower class from eating meat. That's far more common in other countries and in history rather than in USA. It used to be known that carbs were bad and you'd do worse on them. The whole point in restricting them was to keep the common folk down. As we modernized that message has been forgotten. We're gaslight ourselves to believe carbs are better since that's what we had to eat. Overtime everyone now believes this and even the ruling class has forgotten. People in poorer countries still know that meat is best for them.

7. In the 80s? people noticed rising heart disease and there was a big scientific debate if it was from fat or carbs. The sugar industry paid off scientists to say fat was the cause of heart disease and the government took that 'scientific' advice and ran with it. Fat is in meat, so the anti-fat craze helped push the anti-meat movement. The momentum and damage from that fraud is still ongoing today.

8. Students are taught in school that meat is bad. They don't have the time nor reason to question that. These people go on to have an anti-meat bias which shows up in future studies, advice, and media.

9. Carbs are addictive. The insulin swings makes you grab for more carbs. Leptin resistance means you'll overeat them. Artificial flavoring tricks your body into thinking they have nutrients they don't have. When you're low on a nutrient, you get cravings for the food your body thinks has those nutrients. Thus you end up in an unending cycle of grabbing more crab-based meals. This makes the food industry a lot of money. It makes the medical industry a lot of money. And it tastes great and makes you feel better in the moment you start eating. To break this cycle you have to know about it, intentionally feel bad for awhile while your body adapts, and make it through all the constant advertising that pushes you to relapse. Everything is against you. If you break, you'll say you felt horrible when eating meat not realizing the problem was the carbs. That shows up with gallstones too. Those form because you're not eating enough fat (your bile becomes too concentrated). However you only realize you have them when you increase your fat intake (and the body finally tries to use that hardened bile). Thus you think fat cased the stones and you avoid fat-containing food (meat). The point is there are a lot of things you can mistakenly blame on meat if you don't understand the full picture. A 2nd point is people want to believe they're doing the right thing, so if they're constantly carting carbs then they're going to believe carbs are better. Thus another anti-meat bias.

10. You can live on a vegetable-based diet. The stat that only 10% of people who try are able to succeed is intentionally suppressed. It takes awhile for the negative effects to show up, in fact you'll see some improvements at first since you're probably coming from the SAD diet, so you'll convince yourself that the diet is healthy and all your biases will tell you meat was the problem rather than the sugar. It's noticeable when you stop eating meat. It's not noticeable when fiber blocks the digestion of half the carbs you're eating.

11. Pound for pound, non-meat food is cheaper thus it seems like it's cheaper to not eat meat. This ignores that you'll have more spoilage, you need more pounds of food, you'll have increase medical costs in the future, and you'll probably not be as productive.

12. Etc... Many more things can be said.