The playdisc is in mono, allowing for more content to be stored on the disc.
These discs are created for a retail environment or 'background music'. This is how most comercial retail environments work as stero would be of no benefit unless you can make all of the customers stand in specific areas. This is what makes it possible to compress to such an extent. this is taken steps further with ethernet connections with large retailers who have their own gateway and submasking addresses where content can be left in pac files that are automatically picked up in small quanities to supliment exsisting playlists and configurations such as dayparting or stimulus progression