Because you'll need to offer 100% of the technical support. In the times I've tried to use Linux and something has gone wrong, I've done my best to research my problems online. I don't know about the other distros, but I was very disappointed by the Ubuntu forums. There's no support for newbies over there.

*edit*
And I can't believe you keep saying this in thread after thread that the non-computer literate would thrive in Linux. I'm still convinced that you have little actual experience supporting these types of people. I do, and I'm certain that they wouldn't.