According to 'The Tipping Point' by Malcolm Gladwell, Kevin Bacon can be connected to the quarter million or so actors and actresses who have ever appeared in the movies, on average, by 2.8312 steps. Ranked among all actors ever, Bacon only ranks 669th. More 'connected' actors include folks like Martin Sheen, Elliot Gould, Robert Mitchum, Gene Hackman, and Donald Sutherland. The best connected actor of all time was Rod Steiger.
I have no idea either why Kevin Bacon was chosen to be the centerpiece of this game. He is an affable, versatile actor, appearing in many different genres, and he does seem to show up in pretty much every ensemble movie made over the past 20 years. One could argue he was chosen, because the young people who invented this game are simply more familar with him than guys like Sutherland, Hackman, or Steiger.
Spode, I think Cool Dude meant "Honeymoon in Vegas", not "Leaving Las Vegas".