When the Eldar fell due to the birth of Slaanesh, it seems odd on how the Eldar's pride became their bane. When the Eldar pursued their pleasures they were making a power in the warp powerful, Slaanesh is the Daemon god of pleasure and pride.
Most Daemons can bend someone's mind and this power that became Slaanesh could have been powerful enough to bend most Eldars mind to live their lives of decadence to make Slaanesh be able to form. What I think is Slaanesh manipulated the Eldar into giving Slaanesh the power to form!
Hopefully, this'll make sense.
See what you think.