It would have been cool to see this interaction to build the emotion between the Captain and her former pupil but I can see what Rian Johnson means. As he told Entertainment Weekly, the pacing wouldn't have worked. With a ship exploding and rebels on the run, grinding the story to a halt in this moment to let two characters have a little chat would have messed with the flow of the movie. We never saw the coercion that took place in The Force Awakens before Captain Phasma lowered the shields and was promptly and unceremoniously disposed of. Nevertheless, seeing a little more insight into Phasma's psyche to see how she reacts to having that shameful moment revealed would have been interesting. It also would have done a bit to humanize the character, who never got as much screen time as we hoped, before she reverted back to a cold, calculating killer, murdering her own men to save face.