In 1377 DR the then zulkir of necromancy Szass Tam siezed power and killed or exiled his peers, the zulkirs of the other schools. Consolidating his political power within Thay. Today Thay is still magocracy, albeit lead by Tam and with a distinctive focus on necromancy.
Due to their extreme focus on one school of magic Red Wizards often delve deeply into ritual methods for their school. And with Thay's focus on necromancy, Red Wizards have become particularly adept at their favored school's dark rituals.