Charter school study

In response to my recent email in which I said I'd like to see a study comparing students who won the lottery to get into a top charter school vs. those that applied but lost the lottery, a friend at Achievement First/Amistad wrote:

The study we tried to do was to compare the academic results of the kids who got picked in the charter school lottery with those who were not picked. The names of all the lottery entrants from prior years are in the district's hands and Mathematica approached the district to do the work at no charge. The study could have been completed in a few days. The district refused to cooperate.

Gee, I wonder why???