1. Khan was already a major villain in a 'Star Trek' film and this is a universe brimming with dozens of other great adversaries for the crew of the Enterprise to face down.
2. Khan was was so memorably embodied by Ricardo Montalban that any other actor would find himself stumbling in shoes that are waaay too big for him.
3. This is evidence that JJ Abrams and writers Damon Lindelof, Roberto Orci and Alex Kurtzman aren't ready to take the easy way out and rely on characters that every moviegoer is familiar with. Instead, they're picking storylines that will make a good movie versus pandering to the masses.