Was it ever revealed why Pertwee was marooned on Earth?
There was never any mystery about that. From Troughton's final episode, the conclusion of "The War Games"
TIME LORD: We have accepted your plea that there is evil in the universe that must be fought, and that you still have a part to play in that battle.
DOCTOR: What? You mean that you're going to let me go free?
TIME LORD: Not entirely. We have noted your particular interest in the planet Earth. The frequency of your visits must have given you special knowledge of that world and it's problems.
DOCTOR: Yes, I suppose that's true. Earth seems more vulnerable than others, yes.
TIME LORD: For that reason you will be sent back to that planet.
DOCTOR: Oh, good.
TIME LORD: In exile.
DOCTOR: In exile?
TIME LORD: You will be sent to Earth in the twentieth century, and will remain there for as long as we deem proper, and for that period the secret of the Tardis will be taken from you.
DOCTOR: But you, you can't condemn me to exile on one primitive planet in one century in time! Besides, I'm known on the Earth. It might be very awkward for me.
TIME LORD: Your appearance has changed before, it will change again. That is part of the sentence.
So basically they sentenced him to community service.
And the hypothetical season between "The War Games" and "Spearhead from Space" (possibly incorporating the Second Doctor's appearances in "The Five Doctors" and "The Two Doctors") is season 6b.