Here is the skinny.
The Emperor didn't actually trust the Space Marines. He needed super soldiers, but the real Apple of his Eye was always good old regular humans. He didn't want the Space Marines to replace humanity, just to go to war and keep them safe. This turned out to be a good idea since *literally half his Space Marines betrayed him*. Space Marines are infertile and only men very specifically so they cannot breed, and if things get out of hand they can't like splinter off and form some competing empire of supermen. Which kinda happened anyway.
Custodies on the other hand, are pretty much custom made perfection in every way and are absolutely, totally, incapable of ever being disloyal to the Emperor in even the slightest way. They were meant not only to be his personal protectors, but also his companions that he would hang around with.
The whole execution was just shit though, it would have just been better if they had some Tech Priest find a old dusty box of female custodies geneseeds and decided to make some. But it is what it is.
Besides half of fedi is obsessed with mommy dommies anyway so there.
