Because it's unlikely that articles like this one would have been written without the "hook" of Hawking's name. He's allowing a bit of his fame to be reflected onto the project, and I suppose that's a good thing for all involved.
if thats the reason they could get better coverage inviting Paris Hilton to do the showing.
And I imagine that if they could find a decent way to tie her to the project, they would.
Hawking was an easy choice, writing an iconic book with "Time" in the title. A tie to Paris Hilton would take a bit more creativity.