Not enough cross-connection. Dinah's worked with Ted and Booster before and never showed much chemistry or even much friendship with either of them, and Booster and Babs have never even interacted. It's more a straight line of attraction than any kind of web; you get two people who are into two of the three people they're with, and two people who are into one of the three people they're with, and you're not really OT4ing or making your characters real happy. OT3s are a lot simpler and more fluffily appealing because someone's into everybody and nobody's into less than half the triangle.