Learn French: Il a menti, c'est pourquoi nous avons jugé cela inacceptable. - He lied, which is why we judged that unacceptable.

In French, cela is used to refer back to an idea that was just mentioned (in this case, the fact that he lied). Ceci is mostly used to draw attention to a new idea or something you are about to say.

Could we use ça instead of cela here?

Yes, but ça is generally used in informal spoken French. Because this sentence uses a slightly formal structure with the verb juger and the connector c'est pourquoi, the more formal cela fits the tone much better.

Could we say nous l'avons jugé inacceptable instead of using cela?

Absolutely. Using the neutral direct object pronoun l' (referring to the whole previous clause) is very common and means the exact same thing. Using cela simply puts a little more emphasis on the word 'that'.

Is c'est pourquoi the only way to say 'which is why'?

It is the most standard translation, but you will also frequently hear c'est pour ça que in everyday spoken French, or voilà pourquoi (that is why). C'est pourquoi is an excellent choice for written or slightly more elevated French.