Hi, drop tower user here :-) Three drops per day is due to 20 min preparation, 90 min evacuation, 30 min to refill with dried air, 20 min recovery. The is indeed long, which was not anticipate when the tower was first built, this the lack of automatic recover systems. Also, many experiments need to repair/change stuff in their apparatus between drops.
According to Wikipedia, they need 90 minutes to evacuate the tower before every experiment. Does an individual experiment involve multiple (dozens? hundreds?) of falls?
I'm assuming they have a long list of people who want to book time in the tower.