"Carnist isn't a word. What even is this? Just some lame attempt at insulting meat eaters? Ha!"
Don't visit any semi-ironic humor-based vegan community online then, where the snickering insults range from "omniscum" to "bloodmouth" to even more specific ones like "cheesebreather" for vegetarians. "Invertebrates" is a good one on account of a lack of a spine. The list goes on. "Carnism" isn't intended as an insult, and if it were, wouldn't break the Top 20.
Veganism is the rejection of the carnist claim of special pleading, either that humans get magically different rights or that some species get special priviledges. If you are making these claims, you are taking a moral stance such that you are allowing yourself to harm animals. The vegan stance is that this isn't warranted. Furthermore, under any reasonable moral system, veganism is the default position. So there are people that accept this default, i.e. "normal people", and there are people that don't i.e. "carnists", which are the moral outliers. Hence, the use of the term rather than non-vegan.
It's a bit weird that, in any other context, we refer to the person perpetrating an immoral action by the positive demonym, and we usually lack one for those that abstain. Is a non-rapist... a consensualist? What about a non-murderer? Or is a non-thief a... possessionalist? So why does the person that doesn't abuse animals get the name and those that do perpetuate immorality in this case not?