I think it's the curiosity factor:
You see someone looking at something you can't see, and you wonder, "What ARE they looking at?" And suddenly people are willing to pay money to have their curiosity satisfied.
I guess the biscope is similar to one of those viewfinder things. A picture goes by while the music plays, and the man turning the crank makes the pictures keep moving past.
There's a more sophisticated version, too. One where you can look "around the Europe".
The display at the museum portraying 1947 had a biscope as part of its street scene.
Yet even today, their appeal lives on.