4. Gallifrey Hasn't Been Destroyed Yet
By now it's fairly well known that Gallifrey, the home planet of the Time Lords, was destroyed in the Time War--and that the Doctor, to his everlasting shame, had a hand in its destruction. But in the classic series, the Time Lords are still alive and well. Granted, the Doctor tries to have as little interaction with them as possible, but he cannot avoid crossing paths with them on occasion. Some, like K'anpo, actually make useful allies while others, like the Rani and the Master, are enemies that must be defeated. Sometimes the Doctor even visits Gallifrey (although this is not usually of his own free will), and several classic episodes are set there. And let's not forget that his companions Susan and Romana also hail from the same galactic coordinates. The Time Lords' presence was still felt in episodes that didn't directly involve Gallifrey; it was felt even in An Unearthly Child despite being the first episode we meet the Doctor. The Doctor may not have liked it, but he could never fully escape the Time Lords' influence because they were the reason he and Susan fled Gallifrey and the reason he kept traveling and never looked back.