Given that the neighbouring Okinawa World features the (based on online data rather run-down) Habu Park, I'm not surprised that the Aquarium is not too interested in venomous snakes.Okinawa Churaumi Aquarium has/had displayed some sea snakes and sea kraits, but also native species. iZoo was the only place I visited with non-native venomous snake species.
Based on YT videos, I'm also not surprised that neither you nor any other ZooChatter in Japan seems to have visited the Japan Snake Center, despite displaying species like Pseudechis rossignolii.