Partly because of how light it is at 10pm for awhile -- why waste the effects when they can't be seen that well? Also why bother having it the days the park closes at 7/8pm, doesn't make much sense then. It is pretty expensive to run, so that's why it's kept to when the crowds are getting bigger as a balance for the light. I love it personally, can't get enough of it, either watching or working it!
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PO!NT OF VIEW - A Different Look at Roller Coasters
http://www.crosswinds.net/~justmayntz/thrills/