Why Brooklyn used internal magazine armour

Why did the Brooklyn class use internal magazine protection instead of extending an external armour belt over the magazines?

Internal armour better fit the treaty displacement limit, hull shape, and structural weight budget than an extended external belt.

The armour arrangement was not merely a preference between two equivalent layouts. Brooklyn's designers were working inside treaty displacement limits, so protection had to be weighed against every other demand. Hull shape and the ship's internal weight structure also mattered. Extending an external belt around the magazines required more structural work and weight, so internal protection better fit the hull and displacement budget.