User blog comment:Seieireppa/God Eater Z, chapter 1: Arrival/@comment-14812118-20150814001124/@comment-1419424-20150814052958

Legendary Super Saiyan, comparatively speaking, isn't all that powerful. It's weaker than SS3 (probably) but comes with the advantage of nearly unlimited stamina/energy, which is what makes Broly truly fearsome.

As for why no one else could achieve it? Simple; it's a genetic trait possessed only by Broly (and presumably certain of his ancestors).