You can try a novel treatment on those but at the same time are limited by ethical concerns regarding pain and future survival (if the transplant "works", you are now in a tricky situation, as you can't easily do anything that has the potential to make the situation worse... and given it's uncharted territory, anything has the potential to make it worse).
Brain-dead people don't have such limitations. You don't have to worry about causing pain nor shortening potential survival, so you can try things that are likely to "kill" them (cause the transplant to fail, or other issues) and learn from the outcomes.
Fortunately determining brain death is a problem with a clear-cut answer with a clear line dividing “brain death” and “not brain death”. Right?
Part of it is a choice you make when choosing to donate your body for research. There's a chance brain death can be determined incorrectly (though in this situation it's likely the same determination will be used to withdraw life-support, research donation or not).
If I'm ever declared brain dead, I want scientists experimenting on me. That's a much better use than giving organs to just a handful of people. It pushes the salients forward for everyone.