It's always been like that, innit? The Chaos gods are not at all created equal, with Khorne as the major poster-demons and main antagonist towards the Stormcast. Slaanesh has always been playing second fiddle.
Furthermore, I don't think this is the final form of the Hedonites - Slaanesh is still in chains in the Gloaming, and the hunt is still on. GW is probably waiting until Malerion and Tyrion's aelven factions are ready for release to advance the storyline further. A wise choice, in my opinion.