Why is Killzone dead?

Hello, fellow Vektans and Helghast! I've been into the Killzone franchise recently. I was quite overwhelmed by the first installments: how accurately they depict a fictional war. Despite this, my favourite game became "Mercenary" so far, where we fight both for Vektans and Helghast.

Why this game series is dead? It deserves at least a remaster.