Simple
The human interaction, unfolding personalities, and strategy are intriguing. At least on Survivor. That and a healthy dose of T&A. Unfortunately the usually vote off the 'hot chicks' first.
BTW, T&A are the reason men watch "Dancing with the Stars".
Ballroom Dancing? Yeah right, every man's passion. Don't be fooled.
Cable shows are even better. You should watch "Rock of Love" with Bret Michaels. A non-stop parade of slutty wonders. Even the wife gets a kick out them. Of course I only watch when nothing else is on. ;)