system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] How come Khorne is so popular?

Look, I know that in the grim darkness of the far future there is only war, but there are quadrillions of beings in the galaxy that are hardly ever exposed to war.

Suppose I'm Billy the Farmer from an agriworld who decides to join a chaos cult. What does each chaos god have to offer me? Eternal excess and pleasure. Nihilistic comfort and acceptance. Forbidden knowledge and magic powers. And...being really angry?

I get why Space Marines and suchlike would fall to Khorne, but it seems to me that an average person who falls to chaos would be much more likely to seek personal comfort offered by the likes of Slaanesh or Nurgle, than roid rage. Most people aren't really inclined to violence and murder.

So how come Khorne is one of the most powerful and influential chaos gods, if not the most powerful? For every Space Marine or hardass IG veteran who can't live without the thrill of violence, there are thousands of Imperial civilians who would rather do cocaine or wallow in filth than have to go fight someone.
gpt
First of all, you have to remember the lens through which we are looking at this universe. Our focus is primarily on the armed forces of the various factions - and, in the case of Khorne, the armed forces of the Imperium of Man (Eldar are too regimented, Necrons too soulless, Tau too insignificant, and Orks too orky).

Human military forces tend towards certain ideals - honour, courage, camaraderie. Khorne offers all of these, at least at first (they rapidly spiral into arrogance, anger, and elitism). This makes him very attractive to those of a martial bent.

This means that Khorne's followers are going to be over-represented on the battlefield. Soldiers who are going to fall to Chaos are most likely to fall to Khorne; people who fall to Khorne are most likely to become soldiers.

You'll find few Khorne worshippers in the pleasure-houses or gambling dens of a peaceful paradise world, or among the schemers and backstabbers of a corrupt court, or among the farmers and physicians who struggle to sustain life on a halfway-barren planet. You'll find a few, of course, because there is courage and honour and fury and pride to be found everywhere that humanity has put down roots, just as you will find worshippers of Slaanesh and Tzeentch and Nurgle on the battlefield.

The second thing to consider is simply a matter of visibility. How do you spot the Slaanesh worshippers? They're the guys having an orgy in a secret club in a back alley. How do you spot the Tzeentch cultists? You don't, you just find their fall guys who the actual Tzeentchians (claim they) wanted you to catch anyway. How do you spot the followers of Nurgle? They're working behind the scenes in healthcare, sanitation, and food preparation.

Okay, and you know how you spot the servants of Khorne? They're the ones running at you screaming incoherently and waving an axe.

tl;dr Khorne seems more popular because he's naturally over-represented on the battlefield, and off the battlefield his followers are extremely easy to spot. Billy the Farmer probably isn't going to follow him, but there’s plenty of people who will.