I can only speculate, but if the decedent was a member of a racial or ethnic minority, refusing to prosecute the shooter can lead to protests and political problems for the prosecutor. On the other hand, if a grand jury no-bills, then the prosecutor can shrug and point to the grand jury.
Of course, the grand jury process is secret, so the public won't know the prosecutor most likely presented only exculpatory evidence and recommended a no bill.
Politics.