Slaanesh is kind of fundamentally either indulgent (viz. Eldar) or escapist (some terrible hive world). It doesn't offer much in the way of actual power. All the others do, even if in the case of Nurgle it's "come at me, bro, I really don't care".
Slaanesh does offer power, great deals of it for those who know how to use it.
His greatest champions can move faster than mortals can perceive, can see and hear things outside the normal range of human perception, can bend and twist with an unholy agility, beguile those around them with words thick with warp taint and dazzle the mind with a glimpse of physical perfection.
Such a champion can dodge bullets, raise entire worlds up in heretical devotion with but a few moments of speech, are so fair that few can bear to hurt them and bear a physical strength hidden by their frailer facade.
Add to the pile: control over hordes of adoring slaves who will happily die at your command, and more debauched sex0rz than you can stomach.
The problem is ego. Nurgle has none, Khorne's is pretty well-sublimated by bloodlust. Tzeentch has ego but not the kind that pities itself when it loses. Slaanesh though, Slaanesh is all about ego and self love and it's hard to square that gracefully with defeat. What makes them such entertaining and flamboyant villains also makes them the most pitiful beaten adversaries to me. You strike down something of Khorne and you think well, yeah, that's all part of the game plan for them. You strike down something of Nurgle and you go well, yeah, they'll be back because it's all part of the plan. You strike down something Tzeentch and at least sometimes you go well, yeah, that's probably part of the plan too. OR IS IT?!?!?!!?!?1111
But with Slaanesh you strike them down and it's like "Serves you right, you degenerate scumbag." Sometimes you get that with Tzeentch too but I've always gotten the impression failure doesn't really rankle Tzeentch. Sure someone may get turned into a Chaos Spawn in a fit of pique, but by and large they're on to the next scheme.
Maybe I'm just too heavily inundated with HH Fulgrim style spoiled brat writing at this time and it's coloring my perception. But if I'm honest, my sense of contempt for Slaanesh has always gone back a ways. Even after I stopped thinking Khrone was the best thing since slice bread as a teen, I've always retained a special sort of snigger for Slaanesh. Because they do so honestly have it coming. Loken punching out Lucius in a duel is a microcosm of what the 40k universe feels for Slaanesh. It's not a coincidence it's also how I sometimes feel about the Eldar, but to a less extreme degree. Self confident fast-types always leave you longing to land that solid blow and put them on their ass.
People who dive in head-first and don't play hard to get with their own human determination tend to end up as Chaos spawn.
You get the occasional savant/true believer too.
And you know I'm all about my girl...uh...guy? Thing? Slaanesh. Yeah.
Bovine love goes back a long time in human history, don't ya know. Got cold out on the plains at night.