”Conspiracy theorist” is a term used to end discussion. It is invoked to say “you are wrong no matter what you say, because you are a conspiracy theorist, and all conspiracy theories are wrong.”
Is this reasonable?
Have people conspired in the past? Yes. Is it irrational to theorize? No. So what is wrong with theorizing about people conspiring? Furthermore, conspiracy theories have at times turned out to be true. Epstein’s sex trafficking was a conspiracy theory. Northwoods, the gulf of Tonkin, operation mockingbird, fraud in the 1960 JFK win (gasp! Not election fraud!) were all conspiracy theories, or at least take the form of conspiracies theories. Perhaps most powerfully, what was the paradigmatic conspiracy theory? The classic tinfoil hat, schizoid, stretching-yarn-across-the-bulletin board, poster-child for all conspiracy theories? It was that the government was watching you. Keeping track of your movement and your correspondence, listening to your phone conversations - totally nuts right? Until Edward Snowden revealed that it’s basically true.
If people do in fact conspire, it's not inherently irrational to theorize, and also true that numerous events classically dubbed conspiracy theories have turned out to be true, then why should labelling someone a conspiracy theorist be regarded as a discussion-ending smear?
The simple reason is that it’s a euphemism for “dissenting opinion.” You can’t tell someone they’re wrong simply because they disagree with you. As gullible as people are, that one is too on the nose. Furthermore, the left (despite ideological hegemony across all major institutions) maintains the illusion that they’re the underdogs fighting against the man. So for them, the word “dissent” is inherently positive.
This is the function of a smear. The smearer presumes to win the discussion outright by branding the opposing view as one that any right-thinking person knows is just silly and irrational. In this way, the smearer may avoid contending with the specifics of the dissent, as well as distinguish himself from instances wherein he was similarly incorrect (such as the years long Russia hysteria): “Sure, I was wrong. Honest mistake. But YOU were trafficking in CONSPIRACY THEORIES.”