Heavy and light cruisers share light-cruiser and scout ancestry

Did innovation or expense push navies from protected and armored cruisers toward heavy and light cruisers?

Neither directly; both modern cruiser categories largely evolved from the light-cruiser and scout-cruiser lines.

Neither technological obsolescence nor cost alone explains the transition. The interwar heavy cruiser and light cruiser descend mainly from the First World War light-cruiser and scout-cruiser families. The old protected and armored cruiser line is therefore not a direct parent of the treaty heavy cruiser in the way the names might suggest. This lineage matters because role, scouting, and global presence shaped the new ships as much as armor classification did.