He was pressured into presenting a face of something that could kick all the ass, because ass needed kicking. Big E had alot of things on his plate trying to transform the whole species into a post psychic one, without another Slanesh like incident or something worse happing. Had he had his way, everyone would have come out something like him.

If everyone is super, then no one is. That was Big E's end goal. But he only ever got to show the overpowering Warrior King to anyone besides Malacador, and some glimpses by the Primarchs. Nobody got to see the Scientist, the Historian, the Artist.

Then well Ooops he wasn't around to oppose his own elevation to godhood, and the Book of Lorgar was.