I don't have a source for you, but my understanding is that the main reason for the change is that very recent studies have shown that people with vaccines and asymptomatic "breakthrough" infections carry a much lower viral load + viral shedding than previously thought. So even those vaccinated people who may have an asymptomatic infection are really at an extremely minimal chance to spread it to anyone else; they just aren't shedding enough virus particles to make the risk high.