Opportunity cost, opportunity cost, opportunity cost. 2-3m shipments per game is not good enough for a studio that size.
Shadow fall has been helped by aggressive bundling and being a launch title. It would not breach 1.5m without that help because it is not a good game and the Killzone franchise should have been flushed down the toilet at the end of last gen.
Yet here we are, GG got a prime PS4 release slot Cambridge got shunted to Vita and are shedding jobs. What's worse is that Mercenary is a better game as it is built on an all new engine made in Cambridge.
SCEE is just completely fucked up at the moment, management choices have been poor, there are a lot of agendas being played out, chief among them the willingness to let other studios go to the wall to protect the original poor decision to waste so much money on buying GG. Remember that Millennium Inc. (Cambridge), Psygnosis and Evolution were cheap. GG was not and I've heard that it took a lot of upselling by SCE managent to group level management back then to get it done, and it was on the basis that they would turn Killzone into a Halo killer under first party ownership.