"why does he believe it's appropriate for him to appear on the show now? What has changed?"

Your premise that there have been "no relevant developments" is flawed. Maher dedicated much of his following show to a mea culpa. He apologized and was forgiven by at least one leader in the black community (Dr. Dyson). That all took place during the episode in which Sen. Franken decided to appear. That may not be a meaningful development to you, but it was to me and may have been to Sen. Franken.