"I can pulp your flesh and snap your bones in less than a second and without so much as lifting a finger. What is the power of technology compared to that?"
Librarian were banned during the latter part of the Heresy due to the Council of Nikaea. The Emperor invited the advocates of both sides to attend the conclave on the newly-terraformed planet of Nikaea, to determine whether human psychic powers and the practice of psychic sorcery presented a danger to the newborn Imperium of Man. The Emperor eventually ordered all of the Space Marine Legions to disband their Librariums and for Astartes Librarians to return to service as line warriors and never use their psychic abilities again.
After the outbreak of the Horus Heresy, I assume that any Son’s of Horus Psykers in the rank officers would be allowed to freely explore their powers and eventually would become the first Chaos Space Marine Sorcerers.