>>96406580
Interesting. If we speculate a little, it is mentioned that the idoneth, living in the dark depths of the oceans and with turbo-ptsd from slaanesh, have dulled their senses over time. Since apparently the original cythai souls are brighter, we could assume that this lifestyle has dimmed the souls of the idoneth, which makes them less powerful.
That or gw goes "older = more powerful I don't have to explain shit lol"