There are actually good reasons for that. For one:
Convening the special grand jury allowed Willis to settle questions of privilege using only the supervising judge, Judge Robert McBurney, rather than enduring the endless appeals process available to people like Giuliani, Meadows, Graham and others who would not testify absent a subpoena and a judge’s ruling ordering them to do so.
IOW, it was an end run around a lengthy delaying tactic frequently employed by Trump. You may not realize it (you’d be far from alone), but this was the fastest way for Willis to pursue her indictments.