When park attendance is high enough to warrant it. 15 minutes is resonable for one line. Two lines would empty out the queue and many bobsleds may go unloaded and forcing the use of cast labor that could resonably be spent elsewhere.
i was on the ride on sunday witch was the last day is was open and when i rode it in the afternoon i noticed that they had removed the chain lift so there apparently replacing it and as for it being about nemo sorry no! the tomorrowland side has better views
NavyCopSB is right. It has everything to do with park attendance, and also everything to do with parade routing. If a parade is going through close to the time frame the queue doesn't go there so the parade has enough room.