My conclusion from ell.stackexchange.com:
(1) It's raining. This is why nobody is sunbathing now. — natural
(2) It's raining. That is why nobody is sunbathing now. — natural
The choice of "this" or "that" depends on whether raining is understood as figuratively close or distant from the speaker.
To understand it better, I came up with a couple examples. Let the speaker saying them be in Berlin:
(3) It's raining in Paris. This is why nobody is sunbathing there now. — I think "this" can't be used here because though the situation is happening now, it's happening far far away from the speaker (because there's a long distance between Berlin and Paris).
(4) It's raining in Paris. That is why nobody is sunbathing there now. — I think "that" is natural here.
Is my understanding right?
If not, then why not?