My guess would be lack of power generation. It's real easy to power artillery that relies on shells and powder, powering particle beams is not in the cards on that thing.Gotta admit, I'm now surprised that we didn't get a model of Juggernaut carrying an ion cannon instead of tube artillery first. Where's our Juggernaut Mk IV with the ability to say "Fuck you" to anything in its line of sight 8m in the air? The Steel Talons are slipping.
What I've got from the war scenes is that I, and probably large portions of GDI, are done. This is the last conventional campaign we want to engage in. With everything we need to do, the shortage of people to do it with, and Nod's penchant for striking anywhere they can, I don't want another war, and we don't need to fight Nod for the foreseeable future. Calling for peace, truce, or ceasefire, even without knowing Kane's likely course of action, to allow broad mobilization against Tiberium is something I would expect the nascent state department would move forward with.
Our defense spending isn't going to drop off a cliff, many of the transformative technologies in the civilian sector like prosthetics and implants came from military RnD, and we can't trust Nod over the long term. We have to keep investing in our navy, Air Force and other services to prevent the perception of weakness or vulnerability from developing. Even without Nod, nobody should consider us safe from the Visitors as no foreseeable project would render us so.
I don't like fighting Nod when they're scared, when they've accepted they can't stand up to GDI. That's probably the point of their strategic strikes, though. Convincing GDI it's not worth it to root them out all over the world. Something I keep chewing over from these updates.