How do I say this in the nicest way possible?

Sorry for the ominous title. My roommate and I (both incoming freshmen) are on our apartment complex's roomsync app, and since we signed up for a 4x4, we needed two more girls in our group. At first, the only person I matched with was a girl, whom I'll call girl #1, who had 1 roommate, which meant our group would be full. I reached out, and we talked for a few days, and she seemed okay. My roommate was also fine with her. About two days ago, I matched with another girl, whom I'll call girl #2, who also has 1 roommate. She reached out to me and asked to be roommates. I've been talking to her for the past 2 days, and she's also nice, but I have better chemistry with her than with girl #1. My roommate and I talked about it, and we agreed that girl #2 and her roommate are the better choice, but I don't know how to break the news to girl #1, especially since she already sent the request. She also seems pretty introverted, so I don't think she has multiple options like my roommate and I do. For those reasons, I feel like an asshole for rejecting her and leading her on like that, and matching with girl #2 and leaving her on read seems even meaner. She's also nice, and I still want to stay in touch with her since we'll be living in the same building and going to the same college for the next year. I get that this might not be possible. How do I let her know I found another roommate in the least meanest way possible?