Why the Admiral-class hull did not suit all-or-nothing armour

Could the Admiral-class battlecruisers have used an all-or-nothing armour scheme?

Not readily, because their hulls were not designed around that form of concentrated protection.

The Admiral-class battlecruisers could not simply adopt an all-or-nothing armour scheme. The obstacle was the underlying hull design rather than a reluctance to add protection. An all-or-nothing scheme depends on concentrating armour around selected vital spaces, so it must be integrated with the ship's arrangement. Armour philosophies are not interchangeable packages that can be applied late to any hull.