One of the issues is that the centre half position is not a meritocracy at Gerrard's Rangers. People think players should always be picked on form, but the reality is more complicated.
Goldson infuriates the life out of me at times. For a centre back of his experience, he has some pretty bad traits in his game, from letting the ball bounce to backing off. Despite that, he is Gerrard's de facto captain. He's the one tasked with calling the line, squeezing the game, and making long-field passes. He does these things because Katic and Helander can't. That means he will always be cut a bit of slack when he isn't playing well, and why, barring a catstrophic loss of form (I tend to believe he's always been decent enough despite the flaws) he will be in the team ahead of the other two.
It's not some agenda against Katic, or indeed Helander, it's just the way it is. He's been a fulcrum of this team; a team that has been on an upward trajectory for the past couple of years.