Why did Mary think Jesus was the gardener instead of recognizing Him?
Mary failed to recognize Him because it was pre-dawn darkness and because her last memory was His horrific disfigurement from the cross—the blood, the 39 lashes, and the load of sin. Having been healed of that abuse, He looked more or less normal, though still bearing nail prints, so she could not put it together.
Les explains that on the resurrection morning Mary Magdalene did not recognize Jesus for two reasons: it was pre-dawn darkness, and all she could remember was the horrible disfiguration He had experienced on the cross. She had last seen Him with blood streaming down, whipped with the 39 lashes, and bearing the load of sin, so disfigured that Isaiah says He was more disfigured than any human being in history. Now He had simply been healed of all that abuse and looked more or less normal, though He still had the nail prints. Mary therefore thought He was the gardener and could not put it all together. Yet it was the same Christ who was crucified and buried, now in a resurrected body instead of the pre-resurrected body—the same Person, the same Christ.