The only way up to the top of Pyathetgyi is through narrow winding stairs inside the pagoda. So when the sunset is over (or when the tour leaders think it's time to go back to the hotel for dinner), you're likely to be standing in line for quite a while to get down. You might as well just sit up there until it's almost completely dark. Often the best part of the sunset comes well after the sun actually goes down below the horizon anyway.