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Like Beverly says, the reason they were able to survive is because they were together. The kids in the first episode were not ‘together’. The sole reason any of them were even interacting (aside from Phil and Teddy, more on them in a sec) is BECAUSE of Pennywise. They virtually had no connection to each other whatsoever when they went to the movie theater. On the other hand, the Losers’ friendship was based outside of that. Bill, Stan, Richie, and Eddie were friends before and Bev, Mike, and Ben all joined the group under circumstances unrelated to Pennywise. Ben and Mike joined after the Losers helped them with encounters with their shared bullies. Bev helped them get out of a jam. They had a strong connection well before IT first attacked them as a group in Bill’s garage (which I think parallels the movie theater scene, which is that group’s first group encounter with Pennywise). That’s why a big chunk of the movie is just them hanging out together. It shows how they’ve built that connection.

That being said, Phil, Teddy, and Matty’s ‘friendship’ acts as a sort of foil for Bill, Stan, Eddie, and Richie’s relationship before they meet the others. They’re the bullied social outcasts and this allows them to form strong connections with other social outcasts. Bev, Mike, and Ben are just embraced and accepted as part of the group no questions asked. Meanwhile, Teddy and Phil, who are also bullied social outcasts, have to be bribed just to show up to Matty’s birthday party. Also, Phil and Teddy’s friendship isn’t even strong. Teddy tries to talk about Matty and Phil changes the subject and screams in his face that Matty’s dead. But the Losers were following Bill into the sewers to look for Georgie. They’re all super mindful about not implying that Georgie is dead, going along with Bill’s belief that Georgie is ‘missing’ even though it sounds outlandish. It is only out of terror and anger that Richie screams at Bill and tells him that Georgie’s dead. The Losers draw strength from each other and use that to combat the fear, loneliness, and darkness Pennywise feeds on. The kids in this show are emotionally isolated from each other and thus easily fall victim to Pennywise.