...because they can.
And if a musical charges less, often the buyers will ask if it is not as good as the others! A friend worked a ticket desk in hotel lobbies and he told me this was a common reaction.
I realize both of these answers sound flip, but alas, they are also true.
Go figure.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher