
The reason 11 is called yo in craps is for differentiation. Oftentimes, dealers may mishear “11” as “7” because they are similar sounding numbers. To mitigate this problem, they began calling the 11 Yo or Yo-leven. Before it became known as Yo, players used to say “Six five, no jive.”
It's not hard to see that it is a mouth full, so naturally, the game moved towards an easier-sounding phrase to replace the number 11. Another reason why it is called Yo is because players believe it is bad luck to say 7 at the craps table.