"Why not" was probably the thought process for this magical collaboration.
When you have something you love, you find a way to make things work around it. And that was the case for Vince Weaver, a University of Maine assistant professor who has a love of the Apple computers and the Apple II in particular.
But the Source engine is a touch advanced for the power within the Apple II. So Weaver made his own abridged version of Portal in Applesoft BASIC, which you can view below. There's even the iconic "Still Alive" song at the end, although there's a bit of hitching that sours the rhythm a bit.
There's also a browser version if you'd like to try it out. It's a cool creation - and not a bad way to mark the 10th anniversary of Portal, which is coming up later this year.