Here's how Emperor Palpatine (Ian McDiarmid) was so quickly able to lure Ben Solo (Adam Driver) to the dark side in Star Wars. J.J. Abrams' Star Wars: The Rise of Skywalker revealed that Darth Sidious had been the overall villain of the sequel trilogy all along. Somehow, he was able to survive his fall in Return of the Jedi and had just been waiting to execute his final plan to reclaim his place as the evil ruler of the galaxy. Part of his grand scheme was to corrupt Han Solo (Harrison Ford) and Leia Organa's (Carrie Fisher) son, Ben Solo - who would later come to be known as Kylo Ren.
When Luke Skywalker (Mark Hamill) attempted to rebuild the Jedi Order, he took on the job of training his nephew. Unfortunately, the Jedi Master had a disturbing vision of Ben turning evil. Luke attempted to strike him down with his lightsaber while the young Padawan was asleep, only for him to wake up seeing his uncle with his lightsaber drawn, ready to kill him. This motivated Ben to turn on the Jedi and seek help from Supreme Leader Snoke (Andy Serkis) — who was later revealed in The Rise of Skywalker to be a clone, created by Emperor Palpatine to act as his proxy.
Ben's reaction to Luke trying to kill him was somehow justified, especially since his uncle was supposed to be the one to provide him guidance to become a Jedi. Luke's decision also contradicts the Jedi Code, which effectively disillusioned his nephew. The rather puzzling question, though, is why Ben opted to reach out to Snoke first and not his other family members. The Star Wars Book offers a good explanation of how Emperor Palpatine was easily able to get into Ben's head even without having any personal time with him just like he did with Anakin Skywalker (Hayden Christensen) in the prequels. According to this official guide, the young Skywalker was already feeling sad and lonely as it was. Han would go on galactic missions, while Leia was busy with her Senatorial duties, which left Ben with no one to turn to. "This loneliness becomes an easily exploitable weakness, making him vulnerable to the temptations of the dark side," the book adds.
The Star Wars Book also notes that while one might assume that Luke would provide special attention to Ben, given that he's his nephew and a fellow Skywalker, that wasn't the case. The Jedi Master's attention was divided between him and the rest of his students. Couple that with the pressure to rebuild the Jedi Order all by himself, and Luke was too occupied to notice how Palpatine had been corrupting Ben. By the time that he finally caught on to what Darth Sidious had been stealthily doing, it was too late. Without a Jedi High Council like in the prequel trilogy, he didn't have anyone else to consult regarding the matter, so he haphazardly came with a plan that backfired big time.
The Star Wars sequel trilogy had a lot of missed opportunities including this one. The films reveal nothing substantial about Kylo Ren's childhood; there were no flashbacks or even stories about this part of his life before he succumbed to the dark side which could've made him more compelling and his redemption believable. Driver, in fact, had to come up with his own backstory for Kylo Ren to effectively play him, and interestingly, it was quite similar to what's revealed in the book.