The Seifer Gunblade from Final Fantasy is popular because it is a unique and iconic weapon from a popular video game series. It is also a very large and impressive sword, which makes it popular for cosplay and display.
Additionally, the Seifer Gunblade is associated with the character of Seifer Almasy, who is a popular and well-developed protagonist. Seifer is a skilled swordsman, and his Hyperion Gunblade is a symbol of his power and determination.