Board Thread:Warhammer 40k General Discussion/@comment-58.179.157.107-20140511083958/@comment-184.96.176.105-20140604032340

It had to do most likely with how much the Emperor trusted who. The Emperor found Horus first and they fought hundreds of battes together probably before finding another primarch. The Emperor had known Horus the most and therefore trusted him the most.

However the wise thing for the Emperor to do was to allow the Primarchs to vote who they wanted as their Warmaster, that might have left Gulliman or Sanguinius to the role and probably would have put a damper in or stopped out right the Horus Heresy. Of course the Emperor likely believed that choosing the Warmaster personally was the better choice.

And like 210. said, the Emperor created the Astartes, unlike the Thunder Warriors to be both soldiers and political leaders. The Emperor had thought Horus had the best strategic mind and that other, more charismatic primarchs would be left as the high lords of Terra or governers of segmentums.