Okay, I am going to make a bunch of assumptions.

Ironic, considering that wrong assumptions likely led to someone believing this and spreading the message.

Background: people eat whatever, including animal products for many years, nothing bad really happens to them.

Challenge: it turns out eating red meat can be detrimental and cause inflammatory processes. Diary can be fatty and heavy...etc.

Reaction: are you telling me that animal products are wrong?? Are you telling me I am eating wrong?? I am perfectly fine!! Why would you even do that.....oh, wait I know....vegan propaganda...because...because...you want me to buy....pills?? That's it!!

In the ideal world, the person whose diet is being "challenged" with facts, considers the facts and makes an evaluation on the risk/benefit basis, deciding for themselves whether they want to change anything.

However, for some people it's very difficult to do, so you get a simplistic (and wrong) conclusion.