Because in the 16th Century when surgeons were barber-surgeons they didn't have a medical degree. When the College of Surgeons received its royal charter, the Royal College of Physicians insisted that candidates must have a medical degree first. Therefore an aspiring surgeon had to study medicine first and received the title Doctor. Thereafter, having obtained the diploma of Fellow of the Royal College of Surgeons he would revert to the title "Mr" as a snub to the RCP.
Can I has a cheevo?