The problem with Defense Value is that evasion is also mostly treated as Defense Value. So a character that dons full plate and a dodgy lightly-armored character ultimately feel the same when being on the defense.
Yes, the games will differentiate between these bonuses to defense, sometimes one of them doesn't matter or gets disabled etc by certain actions, abilities, spells. But mostly, they result in characters feeling samey.

With Damage Reduction, you have a mechanical difference between characters while playing them. The tanky character will brush off hits. The dodgy character will actively avoid them. This can result in very different approaches to defense and is therefore more meaningful.