Because unless they change the DR/PRR interaction to calculate PRR first, DR second, DR values can be 5 or 30 and will end up having around the same mitigation.
Back when the biggest baddie in DDO was a Pit Fiend who at best could hit you for ~50 non-crit damage, being able to get DR 10ish was absurdly good. Now common wolves in the wild hit you for this much, and bosses hit much harder. Yet, DR is still capped at 15 or so because... reasons?
And the funny thing is that when you have higher PRR, DR becomes more useless, because when you are mitigating 80% damage, being hit for 320 or 290 damage is basically irrelevant.