For years, corporate debt has been rated higher than equivalent government debt, and once the monoliner insurers started to implode, municipalities realized how much of a racket it was, with the ratings agencies using a stricter standard, so that the government debt had to use insurers where the rater’s brother-in-law worked.
Took long enough, and I’m sure that the fact that various Attorneys General were looking into this, and that there was a bill in congress, had nothing to do with the change.