The problem is, a major plot point was made of Zovaal having been building his Mawsworn army by harvesting the designs from the Primus/Runecarver’s memories. Not only is the Helm of Domination in particular of that design, but as Runecarver the Primus shows us how to make Legendary armor empowered with his memories, and it all shares that aesthetic.
Had Zovaal already possessed that aesthetic, he’d not have needed to extract those memories to create it. The design is baked into the Runecarver’s Legendary designs as well, and is tied directly to objects of Domination, also drawn from him. It’s been repeatedly iterated by now that Domination magic and the associated “Icecrown” look come from the Primus, and that aesthetic is shared by Torghast.
Something to perhaps keep in mind is that Torghast may not have always been Torghast. Its form is mutable, and that form is currently under Zovaal’s control. But it might not have always been, as another major story point is that Zovaal achieved his (until 9.1.5, still Maw-limited) freedom of movement by gaining control of the means by which he’d been imprisoned. That could include the tower itself. Part and parcel of confining Zovaal could have included needing to subjugate and transform his own domain - Torghast (or whatever it might have previously been called) - along with him, and so mastering Domination magic could have also granted him control over the tower as he gained control over the literal chains that held him fast.
I.e. Torghast could look the way it does now because it was turned into that in order to make it his prison, just as Zovaal himself was reshaped by the Primus etching runes of Domination into his flesh. It could even be that the tower’s form always reflected its master’s will, and when Zovaal’s will was in turn shackled by the power of Domination, Torghast changed form to reflect that as one more layer of “chains” that would alter its form to foil any attempts at entry or escape. So as long as Zovaal was entrapped, so too would Torghast remain his inescapable prison, but as soon as he was no longer chained, the very shape and function of the tower-made-prison became his to command.
An Arbiter - as he used to be - wouldn’t really need his home to be an ever-changing prison around him, but an overthrown Arbiter who needed to be imprisoned indefinitely (along with any souls who’d perhaps fought for him and might otherwise try to break out and facilitate their master’s escape) could. Just the fact that the tower changes its shape to confound ready navigation and escape falls in line with the idea that it was either created or significantly modified to serve as part of Zovaal’s confinement, which likely accounts for the inclusion of its “Helm of Domination” look.