One semi-cop-out but still somewhat accurate response to this is that the Scoobie Gang is the sociocultural force. I agree with you that this is ultimately something the show is punting on, or wussing out or something, not giving them a social fabric outside of their internal dynamics, but given that those dynamics are resolutely internal, it makes sense that once Cordelia "enters," she effectively shuts out the rest of the world. Xander is the "cool guy" and, for all intents and purposes, the only guy available to her in her "new" circle. But the show does want it both ways, falling on their faces this episode particularly hard, and tends to use "everyone else" as a kind of stock character, rather than allowing themselves to populate the world with engaging peripheral figures.