I think Jessie is absolutely wonderful and was the reason why I first wanted to see the show, but who outside of people who really follow Broadway have ever heard her name? I doubt she's a bigger draw for tourist crowds than Laura Benanti, who's been in 2 really popular show on TV recently, not to mention SoM live.
And as much as I LOVE Laura, I could not see her in that role at all, and am surprised she did workshops for it? Didn't Sara say she knew very early one she wanted Jessie for the role? Like, way back when she saw her in Beautiful?
I think approachability is a big factor here. It's a show you can basically recommend to everyone, and especially to the main Broadway demographic, as others have pointed out. And Sara's name probably helps. Even those who aren't super familiar with her music would have heard her name, even abroad (she's been on the radio in Germany, I knew who she was), so for undecided people that's always a plus, it gives them a way in if they think "Oh, I've heard of her before". And unlike Bright Star for example the marketing managed to highlight those strength and get the word out to the right audience.