The so called scientific world views Crystal Healing as mainly due placebo effect, cognitive and memory biases. Crystal Therapy can also be classified as a pseudo-science under a number of Stephen Carey's criteria.
The other reasoning for not accepting Crystal Healing as a genuine method is that all scientific methods are open to ‘revision’ where as Crystal Healing techniques are usually mired in ancient beliefs, mythologies and other accepted but not proven theories.
This brings up another important point. Usually scientists will study a particular topic if they are assured of large grants and sponsors for their project.
Companies will gladly support them, since they are looking to patent the research findings. BUT in the case of ancient healing techniques, since they cannot be ‘patented’, there is not much incentive for scientists to study them or decipher their hidden mysteries.