Those games are on different hardware, and at a whole different pace. Ace Combat isn't a simulator, and VR racing games don't work in a 3D plane.
This is actually an asset to Ace Combat in VR. The planes in AC use a far simpler flight model. Since it's arcadey, the game doesn't have to simulate complex flight dynamics or weapon systems of the player and other aircraft. The detail that goes into getting the performance characteristics of the planes in DCS accurate is quite difficult and eats into resources. Ace Combat doesn't go that far in depth...unless you really can dogfight Flankers with an F-117.
I cannot comment on DCS because I dont personally play it.
But if you look at the 4gamer screenshot vs the trailer, you can see the UI layout is different.
I dont think the biggest challenge is the fact that the VR needs to be implemented, any engine could do it. But the cost is that, to have something optimized for feature like VR, you have to downgrade lighting/pfx/battle scale etc. In the end I do not know if that's worth it or not. Furthermore, you'd want game system that takes advantage of VR headset functionality, like aiming with the JHMCS headset system. But this raises question of "what are the non-vr user going to do with it?"
Anyway, tl;dr I dont think its exactly hard tech-constraint. They can pull it off if they want. It's more of an cost associated with it. By implementing this feature, how much more other stuff do you have to sacrifice in the name of VR.
I play DCS (in VR no less) and I can tell you the only major issue with the UI is that it was designed around a mouse. Designing (or redesigning) and implementing a UI to work well in VR is trivial. And when you finally do get in the game, it plays pretty much the same. The only real difference is that you have better situational awareness with positional tracking, you can better judge distance and size with depth perception, and you feel like you're actually in the cockpit of a plane. The mechanics are basically unchanged. Granted, operating some of the complex systems in the plane becomes difficult when you can't see the keyboard--like the many keys used for the radar, for example. Compare that to Ace Combat, where you just use the square and triangle buttons.
As for the JHMCS, that's a cool feature in VR, but the old fashioned system of just lining the plane up works perfectly fine in VR too--without any changes. The JHMCS could still work without VR though. Remember that in Ace Combat, if you hold down triangle button, you can lock your vision on to a target. If it's within the effective range and field of view, I'd think you'd only have to fire away. I could be mistaken, but I think Over G on the 360 had something like this as well.
DCS also supported headtracking early on so the pathway to VR was easier to do. Also the ED has offloaded much of the plane development to 3rd party groups to make modules while they are are working on the engine stuff and maps (they still are prepping the flagship F-18 module along with WW2 map and straight of Hormuz). So the work load is different and DCS' engine was also recently remade last year to incorporate VR and still undergoing updates.
It's true that early headtracking might've helped, but it wouldn't have been by much. The DCS engine is pretty old and work had started on upgrading to Direct X 11 years before. VR was integrated towards the end. As such, the game wasn't designed and optimized around VR. This was something that aimed to be a complex and detailed sim in 2D, which is why VR performance sometimes suffers. Even after the upgrade from the DX9 engine to DX11, it's still a very CPU limited game.
So how does this compare to AC7? Part of the issue with performance in DCS is due to how it was designed. The maps are massive and very detailed in contrast to the smaller, simpler ones of AC. You also have to factor in the complex systems of your plane and other aircraft being simulated in this gigantic map. Further, the engine had to work with the limitations and complexities of the old flight modules and features, so Eagle Dynamics was somewhat hamstrung here. Hell, I think the aircraft for the Flaming Cliffs 3 module could even be traced back to the Lock-On days of Eagle Dynamics.
For what it's worth, however, much of the heavy lifting is done in terms of figuring out what features to implement to aid the VR experience of DCS. Right now, the most significant issues are bugs and performance improvements. And yet, in spite of the current limitations of DCS VR, more people switch over to it and don't go back to 2D.