TFA explains in broad strokes why Kylo Ren fell, Snoke got to him and influenced him. When Rey turns the mind attack back on him she says that Kylo Ren is afraid of not being as powerful as Darth Vader, which creates the impression that he's generally been afraid of not living up to the legacy/power of his famous family.
Why Vader became a monster was never important to the OT, because Vader isn't the main character of those films. Lucas manages to convey some depth to Vader with a few lines and a few bits of great cinematography and physical performance from a man in a mask, but never feels the need to go in depth to make his reasons for going down the evil path understandable or sympathetic. This is because in the OT Vader's redemption isn't really about Vader, it's about Luke as a character and the type of Jedi/hero he's grown into. Vader is, ultimately, a supporting character in the OT.
Why Anakin fell was an important story in the PT, because Anakin is the main character and Lucas was trying to tell the tragic story of the rise and fall of a hero.
In TFA why Ben Solo fell isn't super important, what's important is that he did fall and the impact it has on other characters. Kylo Ren is a supporting character, not the lead. He's the monster for Rey to overcome, the son for Han to try to save, and the face of his oppression that Finn has to finally take a stand and fight against.
Why Ben Solo fell and making it sympathetic only becomes important in TLJ because Johnson decided to spend most of the film building up a redemption/romance plot between Rey and Ren, so that he could subvert it in the end to teach all those pretty girls who try to save bad boys a lesson. To do that he, effectively, had to turn Kylo Ren into the main character of the film and throw Luke under the bus to make his redemption look viable because TFA left Kylo Ren in a place where there was no clear path to redemption and no indication that any character involved (most importantly Rey and Ren himself) would be interested in his redemption. At the end of TLJ the Kylo Ren we're left with is very much evil "just cuz".