Mei says Helena called the space stations ARKs, and that they were meant to restore life to the planet. That's consistent with what I've found here, but there's a problem. The Reseed Protocol, the final step of the restoration process, is being blocked by what it calls "Element Toxicity Rating."
Now that I think about it, there are a ton of Element veins out there, way more than I remember. It must have spread across the whole planet. Based on these readings, it even infected the lower layers of its crust. After witnessing what raw Element can do to a living thing first hand, I can see how that'd be a problem.
It looks like the ARKs have some built in tech to reduce the ETR around them once they make landfall, but right now it's just too high for them to handle. Something's spreading it, and protecting it to boot. Those big monsters out there, they're the problem - the titans.