I don't know but maybe It could be for evacuation/breakdown reasons? Samurai for instance is the same in that they won't operate it with less than 15 people. I was once told it was so that if the ride cut out it wouldn't shift to the 12 o clock position (which it does with less than 15 people but instead around the 8 or 9 o clock position, preventing you being stuck in an awkward position if it cuts out.