User blog comment:DirgeOfCerberus111/Adeptus Custodes Vs. Grey Knights/@comment-2253300-20111028201241/@comment-3258762-20111028202741

And, like Sniper said, the Emperor made the Custodes to be the best of the best (other than the Primarch ofc) while the GKs were created since with the Emperor's death they needed something to response to the suddenly violent Warp.