It's pretty clear from a strategic point of view, something that veterans of the Crusades and the Heresy should be very much aware of.

The worlds chaos control in the Eye of Terror are, compared to the Imperium massively limited. Every planet destroyed is one less the Imperium has, but the Imperium has a lot of planets, and to get close enough to Exterminatus one you need to get past a defensive fleet and the planets defences. You'll be expending ships everytime.

The end result is just putting some pace between your supplying worlds and the Imperial front line planets.

Plus of course, Chaos supplying planets are suffering the weird time dilation of the Eye of Terror. For every year of production by the by worlds in the Eye of Terror Imperial worlds might have 10 years or more.

So even with a planet destoying weapon you have to guess they were still aiming to target only specific planets for destruction while taking others.

Which brings me to my bugbear. What's the one time you, as clear thinking Chaos commander (if one can said to exist). Why would you drop on to a frozen Death world, with zero production capability that houses a legion strength chapter of Space marines. DAMNIT AHRIMAN YOU'RE SUPPOSED TO BE SMART! Why didn't you destoy Fenris rather than going in for a close quarters rematch?

To put a "Kick me" sign on the back of Bjorn the fell-handed?

Bear in mind, the wolves are nowhere near legion strenght. And Ahriman attacked while the main chapter's strenght was away.

I imagine the EW weaponry is kinda hard to get your hands on, and most chaos clowns don't have it rattling around in their hulls. I imagine that those that do, mostly use them immediately for coolness, or hoard them for the perfect moment. But more importantly I imagine it's for the same reason they don't try it the slow way via months of macros.

You're only likely to use it when you have a commanding orbital position AND you have totally stalled out on the ground offensive. At least that's always the way I viewed it. It being chaos they are even more likely to want to fight an unwinnable ground war, and less likely to say "This planet is too corrupted by the Imperium to ever be liberated, they must be destroyed." So I imagine it happens occasionally, but not enough for the Imperium at large to fear the slow eradication of all its planets via EW in the same way it fears them being eaten by nids.

Speaking of Dragon Age. I'm not sure how on purpose it is, but the Fade and mages are almost a perfect analogue to the Warp and psykers. The main difference between that the Fade is less obviously hostile, and mages a bit less dangerous, sorta how the warp was like pre Slannesh. So my ideas about both are sometimes interchangeable as well Lynata. The idea of certain places between real space and crazy space being thinner or thicker has shown up in both works a few times. "It's easier to summon demons/do magic here, the barrier is thin." Is a phrase I've seen more than once in both settings.