Probably many reasons but here’s one demo: https://en.wikipedia.org/wiki/Braess%27s_paradox

Every actor acting rationally in their own interest can get to a worse equilibrium than if they were coordinated(in this case, to completely ignore the new edge). There are many other examples of this in game theory, you should look it up.

Another question: they say road speeds increased, great. But that’s a different metric than average drive time. Curious if that also decreased.