The last moment of POPPINS could technically be achieved in many houses, including across the country, as Brody pointed out. However, what I am informing the OP of is that the decision was made, in large part, due to that moment. They wanted it to happen in the New Amsterdam. The main reason they wanted that moment to happen in that theatre, however, never paid off like they thought it would, because the show didn't turn into the big-buzzed success that they thought it would.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle