The short answer to this question is that DA Hunter's hand was forced into convening a Grand Jury.
After months of investigation and significant infighting between police and the DA's office, there was a lot of pressure coming from all directions to convene a Grand Jury. Hunter was hesitant. I'll go further than that, Hunter did not want to convene a Grand Jury. He wanted the police investigators to make a case of evidence collected so far and to prove to him that a Grand Jury was really needed. Significant to note that DA Hunter was at the core of non-cooperation with the investigation, refusing to sign subpoenas, leaking information to the press which portrayed the police in a negative light, and all around delays that negatively impacted the investigation.
The Whites were pushing for a special prosecutor to be appointed and a Grand Jury. The police were pushing for the same, so was the media and the general public. There was pressure for Hunter to be fired for what was considered to be his ongoing obstruction of the investigation. He was publicly accused of prosecutorial malfeasance. He stalled as long as he could, but when it became clear that his job was in jeopardy, he finally agreed. There were two incidences that also served as the final push. The resignation letter of Steve Thomas which roundly criticized the DA and his office for their handling of the case, as well as the letter from Priscilla and Fleet White which also brought attention to DA Hunter's consistent delays in making a decision to convene a GJ as well as their dismay at the departure of Det. Steve Thomas who left as a result of the meddling of the DA and his office.
Sadly for the case, by the time the Grand Jury was in place so much time had been lost. The subpoena powers were diminished by the fact that some records were no longer available, memories had faded, etc.