Actually, I'm surprised that it's taken so long for this idea to be tried.
LSD was the first of the serotonin-modification drugs to be discovered; and apparently the most potent of them. The problem with LSD use in the '50s and '60s was that the doses were so high that the users went off on psychedelic trips. Serotonin modification drugs developed later, starting with the SSRI family such as Prozac and its derivatives and work-alike compounds, turned out to be very valuable in treating depression (although they have their own side effects). The idea of switching back to the original serotonin-modification drug, LSD, but using it at a dosage that doesn't cause the tripping, always seemed like an obvious approach to try.