We know the story of E.T. being left here on Earth. What we don't know is what happened on the parent ship.

Alien A (on display screen): What do you mean our son got left behind? Where did you leave him?

Alien B (in the ship): Well, that's not easy to answer. We made a lot of stops...

A: Do you even know which planet?

B: Well....

A: You don't even know which planet!?!

-- big pause... B is saved by the bell -- an alert flashes on another screen

B: Good news! Our son has sent a beacon message. We have his location and we'll pick him up shortly.

A: Argh! By the time you reach him, it'll have been several Earth days... Who knows what those idiots there will have done!

B: Sweetie; I'm sure our son will be just fine. You know how resourceful he is...

A: Don't you "sweetie" me. You go get our son! And from now on there'll be no more of these space .. misadventures! He should be back here in school .. with his friends .. and safe!

And this is why there was never a sequel.