Most likely because the date selected does not qualify for free shipping. Push out your delivery date and click the check button. Your options should update.
Remember free shipping requires 9 business days in addition to production. So, if you are ordering 5000 pieces or less your production time is 5 business days + free shipping 9 business days= 14 business days. Keep in mind that weekends and holidays do not count as production days.
Free shipping is only available on orders of 10,000 pieces or less, being shipped to one address within the continental United States.