Because there is nothing like "defending" belt in combat sports. Everytime you fight for world title and you're awarded with new trophy. Just like olympic games, there's no "defending" gold medal etc. There's no difference between gold medal and belt, symbolically, both are thropies, but it's more obvious for belts (instead of head of your foe). Titles and belts holds no real value sportwise in the UFC or boxing. It's just promotional tool. Personally I like fighters with certain styles, something like with video games, you got your favourites in Mortal Kombat for example, same with fighting, it would be painfull to jump bandwagon of every fighter with belt, because some of them are simply boring to watch.