I think part of the initial growth of the defense ability was due to a popular Japanese pro player, and then many players bandwagoned as they noticed the rise in popularity.
Aside from popularity there are a few things that are required for a weapon to be able to use a defense build effectively and .96 gal Deco checks off all the points.
1. Range to out space other weapons and effectively engage Chargers.
2. Not heavily reliant on specific abilities. Defense requires half of a build or greater to be effective.
3. In action enough to be shot at often and would benefit from extra time obtained by surviving extra shots.
Most other weapons aren't able to fit into those categories. (I've been using Slosher Deco recently and it works nicely with defense as well but I've never seen another Slosher Deco run with a defense build.)