Hi Ron,
I think your premise is anecdotal. You've apparently seen lots of demo trees die and so you ask your question. However, I've seen only one of the two dozen or so demo trees I've seen die - less than 5% of them. I or others might well ask "Why do almost no demonstration trees die?"
So, I think that this question has to have a basis in one's own experience and is not necessarily one that is true for all of us - no matter each of our answers.
Given that a demo tree dies, it has to be due to any number of factors; poor skill by the artist, poor condition of the tree beforehand, lack of species knowledge of the artist, poor aftercare, poor weather after, unforeeable events, etc...
Anyway, interesting question, just not necessarily applicable to everyone's experience.
Kind regards,
Andy Rutledge
zone 8, Texas
http://www.bunjindesign.com/bonsai/