The treaty expired because all of the parties to the Second Treaty of Denver rejected several fundamental elements of it when they refuted Ghostwalker's claim over Denver; Aztlan's lack of a role in Denver or much of anything in North American affairs; California's, Caribbean League's, Quebéc's, and Tír Tairngire's recognition and roles (or lack thereof) by the parties to the First and Second Treaties; and a bunch of other stuff that was probably going to be in the Third Treaty — megacorporate influence, including the STC possibly agreeing to the Business Recognition Accords, etc.
Saying it expired was shorthand for "the signatory parties withdrew from the Second Treaty and when they stopped all official negotiations for the Third, there was no longer a legal agreement in place and so the Second Treaty of Denver is officially dead." As the material in Clutch notes, that hasn't stopped the NAN, CAS, and UCAS from continuing to operate under the norms that were established under the first two treaties, but there's nothing to be enforced if they do violate it by, saying, disregarding the sovereignty of their neighbors.
Hubris. Woops.