Some Great Questions Electraus
What makes the Japanese artists' trees so much more worthy of praise?
Japan is generations ahead of the US, many Bonsai nurseries and professionals have been building on the technique, craft, developing trees and overall standards for generations. Their standards are set very high, there's more money involved in purchasing trees and getting trees into high level Bonsai exhibitions. Japan has also had the time and knowledge to select for small leaf size, develop branches in proportion to the trunk and has higher quality pots and stands.
Yamadori or Collected conifers in the United States may be some of the best Bonsai material in the world. IMO, our best collected conifer trunks could be considered better than the best in Japan. However, their yamadori have been Bonsai for many more years on average compared to ours which is a big advantage. In terms of Deciduous and Broadleaf evergreen, Japan is many decades out in front of us as the best specimens are usually grown from seed, cutting or airlayer and take a min of 20-60 years depending on size of the final tree.
Why aren't the bonsai heavyweights outside of Japan referred to as 'masters?'
I've never liked the term "Bonsai Master." I think of Jedi Master, Dungeon Master, DoJo Master and other Corny mystical things with that term. Can you truly master anything? Does Master mean to know everything possible about the subject as in there's nothing you don't know?
I think a better term is Bonsai Professional. There are many high level Bonsai Professionals in the United States and maybe a couple Westerners practicing Bonsai professionally in Japan who could rival many Japanese professionals in Japan based on technical skill(Peter Tea, Ryan N, Michael H, Tyler S, Bjorn, etc.). However, there are many more Bonsai professionals in Japan compared to the US. It's hard to compare each Bonsai professional overall and better suited to compare each pro on technical merit in a specific category/subject. For example-maybe Ryan N is a 10/10 on Developing RMJs, but a 6/10 on most Deciduous, maybe Peter Tea is a 10/10 on scion grafting, but a 5/10 on Display(just examples not where I actually think they rank).