The problem with moving from 8 hours to 6 hours is that there will always be workers willing to do 10 hours or even 12 hours. These workers end up producing 50-100% more with about the same overhead. If you have a worker that will work 12 hours, you only need to pay for one health insurance plan as opposed to having to pay for two insurance plans if you need to get two 6-hour employees.

On top of that, the 12-hour employee is accruing experience at twice the speed of the 6-hour employee. For some fields, putting in the additional hours can make the employee willing to work more hours more productive on a per hour basis. Obviously this doesn't apply to all professions, but for many professions it does apply to some degree.

You'd basically have to outlaw working more than 6 hours to keep overachievers from taking all the good jobs.