We lost essentially for two reasons on the night and (arguably) two reasons that existed beforehand, but the latter inform the former.
On the night:
1) Genge and Sinckler were pathetically overmatched and had their heads pushed out their backsides
2) Pollard is a better place kicker than Libbok
More generally:
1) RSA are a better team with better players
2) Erasmus is a far, far cannier and more ruthless tactician than Borthers.
Our geriatric starting props at least achieved parity in the scrum, but their replacements were given an absolute touch up. It was galling to see as an Englishman, but, as something of an unreconstructed set piece purist, kinda heartening to see the grunts have such a telling effect on the outcome.
And dear old Rassie, the thinking person's supervillain, gives the impression he'd sell his first born into slavery for a result. His call to hook the silky Libbok for the more prosaic and functional charms of Pollard after only half an hour was both completely ruthless and utterly correct.
Borthers, by contrast, is one of nature's nice guys. One suspects he doesn't have that level of cold eyed detachment in him. The plan was always to pull Cole and Marler at ~50-55 minutes and he just couldn't bring himself to punch up a change of tactics on the fly.