Übersetzung - Latein-Englisch - Quare omnes isti libri vocentur libri prophetales.

This is nearly perfect.
But as "vocentur" is in subjunctive, I would change the verb a bit:

"Therefore/That is why* all these books could be called prophetical books".

* Both are possible here.

What about "should be", which would fit in better with the context?

Coniunctivus has got many functions in Latin, jairhaas. It can express possibility, wish, supposition, order...
I've proposed to translate the sentence by "possibility" (coniunctivus optativus), because,without context,it seemed to be the best option.
Anyway, I think that "should be" is possible here too(it would be "coniunctivus iussivus" expressing an order...).

Thank you, Aneta!

You're welcome.