The initial plans were to create a camera recording at 60 FPS, but this was doubled when it was decided that it might make fast moving objects a little blurry – and it’d be unfortunate if the developers went through all that work only to produce an image which looks like you’ve forgotten to put your glasses on.
A frame rate of 120 per second is huge; cinemas tend to show film footage at 24 FPS, which is exactly one fifth of what the new camera is capable of.