I think because there are certain details to consider. The World Forger himself is Sixth-Dimensional at least, since he and the other Monitors simply reconstruct in the Sixth Dimension if killed, and are the only three beings able to access the Sixth Dimension under normal circumstnaces.
On the other hand, the Multiverse that was shattered was created by the World Forger to replace the current Multiverse. The Multiverse itself wouldn't be "sixth-dimensional"; just its maker and the realm it was made in.
Furthermore, there's the issue of terminology and how dimensions are defined in DC Comics. We know Mxyzptlk is a Fifth Dimensional Imp, but the Multiverse is comprised of dimensional branes and bulks, as shown
on the DC Multiverse Map, so the Cosmology is, in part, based on the Brane Cosmology model of String Theory. Bleedspace, the Sphere of the Gods and Monitor Spheres are all Bulks containing Brane realities (Bleedspace is even called an all-encompassing Bulk).
We also know that the New Gods and their worlds reside in higher-dimensional space as well, beyond regular space and time. See here at 21:00 : -------------------------------------------->
The TRUTH Behind DC's Most Infamous Villai
The question is where the Sixth Dimension actually lays in the dimensional hierarchy of the DC Cosmology.
