Does Evidence From Clinical Trials in Psychopharmacology Apply in Clinical Practice?

"What are you doing?" asked the police officer.
"Looking for the coin I dropped a little while ago down the street," replied the man.
"Why are you looking here instead of down the street where you dropped it?" queried the police officer.
"Because this is where the light is shining," answered the man.

Just because the light of research shines on a population of patients in a clinical trial doesn’ t always mean it will illuminate patients in clinical practice. Why is this so?