Because GJJ was transmitted effectively. Ueshiba was a poor teacher and most of the time nobody knew what he was talking about. Plus, his son deliberately changed the techniques to make them easier.The issue is people can still use Gracie BJJ effectively today. And so don't really rely on Royce to flagship the style.
Plus, the core of his method (which is basically Daito Ryu), that is his body conditioning techniques, was seldom taught, even more rarely understood and almost never practiced to any significant degree. After years of research I've finally found someone that showed me some stuff, so I would need to train really hard for a while, then test it out in a competitive setting. But my progress has been painfully slow, to be honest, and my time is not infinite.