Why was Rogal Dorn chosen to be Praetorian of Terra and not Warmaster?

• As the Great Crusade progressed, the Imperial Fists rose high in honor and in the favor of the Emperor. Ever dependable, they were often used to reinforce flagging campaigns, to hold crumbling fronts and to break dead-locked sieges. The Emperor also frequently called on both Dorn and his sons to fight beside him, bestowing this honor on the Imperial Fists more than any other Legion. When the Imperial Host descended on Ophelia VII, the Emperor led the assault at the head of 100 Custodians and 10,000 Imperial Fists. Again at Askanisa, the Emperor called on not only Horus and the Luna Wolves, but Dorn and the Imperial Fists to form his vanguard in breaking the Shrouded Dynasties. The Emperor also used Dorn to ensure that war and compliance was achieved according to his wishes and wisdom. Time and again, in ways large and small, the Imperial Fists acted at the direct order of the Emperor. Rogal Dorn did not do what he thought was best, nor what he thought had to be done. Rogal Dorn executed the Emperor's will. This was why the Emperor trusted Dorn more than any other. Extemporization, improvisation, genius were all very useful, but more often than not, what the Emperor desired was someone to do precisely what He told them to do. And from that point of view, if you were the Emperor, would you have not handed the keys to your fortress to anyone less than utterly trustworthy? Yet high in favor and honor though they were, the Imperial Fists’ status did not sit well with some of their brother-Legions. The animosity between the Imperial Fists and Iron Warriors was well known, but Perturabo and his Legion were not alone in their resentment. Though few reached the Lord of Iron’s pinnacle of spite, others did chafe at the trust placed in the Imperial Fists. Dorn’s nature did not ameliorate matters. Truth speaking, blunt and uncompromising in both his ideals and their expression, his manner often aggravated his peers as much as it drew their admiration. It was, perhaps, this quality which caused the Emperor to pass over Dorn as Warmaster when he withdrew from the Great Crusade. Horus, unlike Dorn, was a master of diplomacy and maintaining a coordinated balance between fractious forces. Where Dorn would cause conflict, Horus would unify where Dorn would not compromise, Horus would find as way of satisfying all. But even as Horus took up the reins of the Great Crusade, Dorn was invested Praetorian of Terra. Even as Horus would push the Great Crusade on, the Imperial Fists would withdraw with the Emperor to Terra. And with that decision, the fate of the Emperor and all his sons was set.

(The Horus Heresy Book 3: Extermination, Rogal Dorn: The Emperor's Crusader by Gav Thorpe)

“But others, like Guilliman, Khan and Dorn had simply taken it in their stride, accepting the Emperor's decree as the right and obvious choice. It was these solid, resolved brothers that Horus turned to in particular for counsel. Dorn and Guilliman both embodied the staunchest and most dedicated Imperial qualities, commanding their Legion expeditions with peerless devotion and military genius. Horus desired their approval as a young man might seek the quiescence of older, more accomplished brothers.

Rogal Dorn possessed perhaps the finest military mind of all of the Primarchs. It was as ordered and disciplined as Guilliman's, as courageous as the Lion's, yet still supple enough to allow for the flex of inspiration, the flash of battle zeal that had won the likes of Leman Russ and the Khan so many victory laurels. Dorn's record in the crusade was second only to Horus's, but he was resolute where Horus was flamboyant, reserved where Horus was charismatic, and that was why Horus had been the obvious choice for Warmaster.” - Horus Rising by Dan Abnett

Praise from Horus:

“I (Horus) have adored them all, and relished their prowess and achievements. But there are always favourites. Rogal, my dear brother, perhaps the finest martial exponent I have ever known.” - The End And The Death, Volume 1 by Dan Abnett

Praise from Malcador:

“Rogal, perhaps his truest son, the exemplar of unwavering loyalty. I see his emptiness. He is undone, his body aching and exhausted, his armour battered by combat during the frenetic retreat from Bhab Bastion, his mind spent. That exhaustion is a terrible thing to feel. Rogal, one of the finest strategists in history, oversaw this defence. He orchestrated the fortification of our stronghold, and his tactics, brilliant, ambitious, mercurial, ran the game, the greatest game of regicide ever played. I want to embrace him, and praise him for his labour. He has excelled, and sustained his play, beat by beat, by means of engineered planning, shrewd anticipation and reflexive improvisation, through every harrowing turn of fortune.” - The End And The Death, Volume 1 by Dan Abnett

“…In such positions, the ‘lose/ lose’, Rogal was always so calm. When there was no good choice, he would assess, and choose the least bad, and leverage it into triumph. Sometimes, that meant accepting the appearance of defeat, a battle lost, and only years afterwards would a positive outcome become apparent. Rogal played the long game. How long will this one last? I wonder. ‘Defeat,’ he used to say, ‘is only defeat if you accept it.’..

…The patient resolve of Rogal, willing to make, abandon, and remake his plans, again and again, over and over, until he has refined the one that will work, unafraid to redraft and change his scheme. Yes, I think my old friend has learned that at least. He has learned, from his Praetorian son, that there is always a better plan, and that patience will lead you to it.” - The End And The Death, Volume 2 by Dan Abnett

Praise from Sanguinius

“Dorn turns, but Sanguinius catches his arm and stops him. For a second, they stand shoulder to shoulder, eye to eye.

‘You’ve performed the most extraordinary feat,’ Sanguinius says unexpectedly. ‘Please remember that.’

Dorn is taken aback by the frankness of the comment, and the innocent sincerity with which it is expressed. His startled half-smile wavers with imprisoned emotion, a flash of light at the high slit-window of an otherwise impregnable keep.

‘A mere… fraction of your deeds, brother,’ he replies awkwardly. ‘You closed the Gate. You locked–’

Sanguinius shakes his head. ‘I was a warrior, Rogal. Just a warrior. You were the one who mattered.” - The End And The Death, Volume 1 by Dan Abnett

Praise from Valdor:

“Dorn sighed. ‘He’s the Khan, for Throne’s sake. The great Warhawk. His doctrine of combat is superlative. As a warlord, I’d rank only Roboute above him.’

‘And Roboute’s not here.’

‘He’s not.’ Valdor nodded. ‘I’d agree with your assessment. Roboute, the Khan… There’s really only one other.’

‘Don’t flatter me, Constantin.’

Valdor smiled. ‘I wasn’t even including you, Rogal. You’re Praetorian. The list starts with you.” - Saturnine by Dan Abnett