Because the unchastity of a Vestal virgin was a crime against the gods, her case was judged by the pontifical college rather than the Comitia Curiata, which usually presided over criminal trials (Cic. Leg. 2, 9, 22). The collegium pontificum consisted of a rex sacrorum, multiple flamines, multiple pontifices minores, and a single governing pontifex maximus. A body of great power and prestige, the college exercised ultimate jurisdiction over all matters of religion. By performing public rituals, establishing rules for private worship, and administering religious law, it acted as an intermediary between the Romans and their gods.
It’s also worth remembering that the Vestals were also one of the collegia – so they too could be part of the panel that made up such a court. (Though we don’t hear much of them.)