I can't tell you why, but the fact that LSD is a schedule 1 drug with a strong stigma and that it's a drug that can't be patented will severely hamper any study efforts.

Some current theories re: microdosing efficacy that I've seen are that people who report benefits from microdosing (beyond placebo) are inadvertently self medicating ADHD symptoms, which seems plausible, given LSDs stimulant effect and 5HT2A affinity.