"Come to the Darkside....We have cookies". I'm not sure where the quote came from or why but that's what is said.
Some of the more common reasons for the change.
1. Better traction...wet or dry
2. Longer tread/tire life
3. More comfort (for passenger)
4. Cost of tire vs. MC tire (see #2 as well)
And there's plenty more. Go to www.mcdarksiders.com if you would like more info. It's a site dedicated to darksiding.