In order to allow the camera to make less noise, the speed the mirror is moved out of the light box and/or the speed the shutter is reset for each shot is slowed down. This increases shutter lag and decreases the maximum number of frames per second.
Sometime the maximum frame rates is more important, regardless of the noise produced by a camera. I doubt anyone will notice in a professional sports stadium full of cheering fans. In situations such as that, why slow the maximum speed of the camera down when no one cares about the noise?
The various Canon models with Silent Shooting have different options:
The 5D mkII and the 7D have Live View only modes that include Mode 1, Mode 2, and Disable. Mode 1 allows for continuous shooting in Live View at a reduced frame rate. Mode 2 allows for only single shooting and doesn't reset the shutter curtains until the full press of the shutter button is released. Disable is for use with Canon Tilt-Shift lenses (and presumably any other lens with a manual aperture control on the lens that would mount on an EOS body).
The 1D mkIII, 1D mkIV, and 1Ds mkIII have no dedicated silent shooting modes in Live View, but do have a silent shooting single mode when shooting via the viewfinder. The only way to shoot continuously with the mirror locked up is to use the regular continuous shooting mode combined with Live View.
The 1D X has single: silent shooting but no continuous silent mode when shooting through the viewfinder. It also has the Mode 1, Mode 2, and Disable options in Live View.
The 5D mkIII includes the same Live View silent modes as the 1D X, and also has a silent mode single, and silent mode continuous setting when shooting via the viewfinder. In silent mode continuous, the max frame rate is reduced from 6 to 3 fps. In this mode the mirror still moves up and down between each shot as it would in normal continuous shooting mode. Depending on the settings selected, the camera will also re-meter and refocus between each shot.