Why Hood did not receive all-or-nothing armour

Why did the Royal Navy not adopt an all-or-nothing armour scheme for Hood?

Designers still feared high-explosive damage when the choice was open, and the ship was too advanced to change once that concern receded.

Hood's armour differed from the all-or-nothing approach used in the Lexington design for two reasons. British designers still worried about the effect of high-explosive shells on unarmoured portions of a ship when changes remained practical. Experience and testing later weakened that concern. By then, Hood's construction was too far advanced for a fundamental redesign.