Question: Is it ever at all stated why the Doctor left Gallifrey?
Tiger Left, Tiger Right - S2-E6
Question: The episode first aired October 1964. How could a Ford Mustang be seen in the episode if that car wasn't available until April 1965?
Answer: Production vehicle models aren't in sync with the calendar year. The '65 Mustang began production in March 1964 and first sold in April 1964, before it was "introduced" the following year. I don't know which model was seen in the episode, but the 2+2 fastback was sold in September 1964. The 1964 film "Goldfinger" uses a 1965 Mustang as part of Ford's product placement. Basically, in the 1930's, FDR ordered automakers to release vehicles in the fall of the preceding calendar year "as a means of facilitating regularization of employment in the industry." Now, automakers can release new models as early as Jan 2 of the preceding year.
Chosen answer: It's principally related to the Time Lord's strict non-intervention policy, which stated that they should only observe events, not interfere in any way. The Doctor staunchly disagreed, believing that they had a moral duty to protect the universe from evil. That, combined with his own wanderlust and desire to see and experience the wonders of the cosmos, was what ultimately drove him to steal the TARDIS and take up the life of a renegade.
Tailkinker ★
Yes he has mentioned before why he left.
Dan23