🤷‍♀️ I assume it's just historic. Off hour trading was presumably so that big institutions, like fund managers, could do large trades in big blocks without rocking the market hours. so we are told. now that Wall Street is looking for every single penny, some retail brokers allow you to trade for a couple of hours before and after the market just like institutions. (after signing a contract)

off hour trading is more volatile (for retailers) as the volume is different, usually lower, than day hours.