Because they were used in the poker room. When the poker room originally re-re-re-opened it was in the pool area in a temporary structure. This was when they were playing limit games such as 20-40 or 30-60 and had these chips for use. By the time it moved to its current location they had changed over to a 1-2NL or 2-5NL games and no longer used the $10/$20 chips. Very few poker rooms still use them, Wynn and Bellagio do get some mid-limit games and use the chips.