Ummmmmmm...cuz you weren't there that day, so they didn't let you design it?...
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Sorry vette, i have no control over sarcasm...
The real answer is: because there is too much travel for a standard U-joint to handle.
Here's a good overview...http://www.google.com/url?sa=t&rct=j...XlhRgu-YdXCv7g