See, the problem was ease of access.
Every race knows disease. Every race knows war. Every race knows ambition. And in many cases, you can have the same race doing all three at the same time in large amounts. Think of the Mongols; they brought war everywhere they went, along with all the political aspects thereof, and used the black plague as a weapon. They're feeding three out of four chaos gods at once.
Now, think of decadence. You can't spend all day enjoying yourself if you're dieing of plague. You can't dedicate yourself to the arts if you're in the middle of a war. You can't eat, drink and be merry if you'll be assassinated in a coup if you let your guard down. Decadence requires peace and stability, while all the others require only warm bodies.
As a result, the vast majority of sentient beings couldn't contribute to Slaanesh because they were too busy contributing to the other three. Only when one race stomped out the competition, got disease under control, and formed a stable empire, did they find the time to debauch themselves enough to contribute meaningfully to the formation of Slaanesh.