Coming back is something the First Doctor said he'd do one day, but unfortunately his dementia set in and so he forgot to return. Yes, he forgot to return. (*holds and strokes lapels*)
I doesn't really matter though, I mean he picked up Vicki immediately afterwards who was infinitely more GOAT than Susan anyway.
Until he does come back though, there must be no regrets, no tears and no anxieties. You must go forward in all your beliefs, and prove to him that he is not mistaken in his.
Oh wait, he did come back in the end? Oh cool...
Shame it's not canon.
Still, a lot better than the crappy John Peel novel where he also came back.