The reason is because the producers, in a bit of brilliant casting, brought the legendary film actress Ruby Keeler out of retirement after almost 30 years for a small co-starring role. Keeler was never considered a terrific actress, but she had a charming, endearing personality that audiences loved, and they loved her tap-dancing. She was best-known for playing the ingenue in the film 42nd Street. You know the famous scene. "You're going out a youngster, but..." oh, here it is --
A friend who did see the show in New York said that when Ruby Keeler made her entrance, the electricity and explosion in the theater was magical. She still had the affection of the audience, and the history behind her appearance only added to that. Mind you, I was never a particular fan of Ruby Keeler. But I do like history and dramatic moments.
Happily, there's footage of her big number in 42nd Street, which was done at the Tony Awards. (Even though she wasn't the star of the show, the producers certainly knew who to feature.) And even though the audience there knew what to expect, and probably most had even seen the show already, their reaction is explosive, nonetheless.
But then, whatever her limitations as an actress even in her prime, and even though at this point in her life her best tap dancing was decades passed...boy, is this a joy to see. And it's no tossed-off moment based on using her name, but she throws her self into with everything.
(By the way, know that after she finally leaves the stage after what appears to be her exit, it's not. Keep watching. There's more.)