The answer is because Hamilton was the biggest musical of the last decade that managed to not only bust open Broadway but entered the public Zeitgeist in a way that no one had really expected.
It's single-handedly started Hollywood's current obsession with revitalizing the Hollywood musical and directly led to the creation of the wicked musical..
Within the Disney canon, he not only sold from Hamilton but produce their biggest modern-day soundtracks with the first Moana and Encanto, so in a very real way he's been keeping the Disney musical of float.
@Screamer_2 it's more so Miranda is kind of the face of modern theater, so when people say they need to make things more theatrical he's kind of the go-to guy.
I think the big dichotomy of somebody who keeps up with Broadway as opposed to someone who just watches the occasional animated musical is that the Broadway that you know from the Disney Renaissance was already being phased out on the stage by the time those films debuted
Rent single-handedly caused a bunch of composers to move past the more operatic / classical sound of Broadway and into this new contemporary sound that we see and things like spring awakening, in the heights, and wicked were the things that solidified how it sounded.
That's why regardless of who they get to write music for them it's going to have that "Poppy" sound because that's just been Broadway for the past three decades and Disney's only now catching up to it.