Why the application process for the choice program is divided into two different tiers for public and nonpublic students?

Under the Interdistrict Public School Choice Program Act of 2010 18a:36b, it states:
“If there is an opening in a designated school of a choice district and there is no student who is enrolled in a sending district who meets the attendance requirements of this subsection, including a student who has been placed on a waiting list based on a lottery held in the choice district, then the choice district may fill that opening with a public school student who does not meet the attendance requirements of this subsection or a nonpublic school student.”

In order to comply with this statute, districts must first fill their available seats with students who have attended a public school for one year.  This is the Tier One enrollment cycle.  Then the district may fill the remaining seats with nonpublic school students during the second, Tier Two enrollment cycle.