The Mark VII armor core is almost identical to the Mark V(B) armor core, so what the point of using the Mark V from Halo Reach when its chest piece is almost identical to the Mark VII ? Honestly I think that 343 should have used the Halo CE Mark V as armor core.
Mark VII looks more like Mark VI than Mark V(B), with the exception of the chest. And Mark V(B)'s chest was already very similar to Mark V.
The default core is basically a blend of classic Halo armor.
CE’s armor core is unique, but it’s a product of its time. Replicating the look of it without appearing clunky would be difficult, which is why it’s just easier to shift more towards Reach’s armor design.