Kecha blademaster armour has a base defense of 16. By base, I mean before you invest any armour spheres (those make a pretty big difference). While Gypceros's is lower, it provides skillpoints into arguably more useful skills than the Kecha armour does. Meanwhile, Tetsu's has 16, but that is raised further by the skillpoints it has the skill 'defense up'. That's why it's so popular.