They also specifically said that one squad in game exclusively having reduced damage modifier instead of more received accuracy is inconsistent and confusing so that unit had it removed last patch, but here we are with grens.
Not being necessary for the unit to function and having no justification while also not being communicated to the player in any way whatsoever. Uniformity was a small part of that, but shocks show that we're fine with a unit being inconsistent with others as long as this inconsistency is important to the unit's theme, is communicated or even just hinted to the player in at least some way, and actually has a reason to be a part of the unit (this last point being the most important). Shocks' armor fit his all of these things. Ranger damage reduction fit none of them. As someone else pointed out, Ranger tooltips/descriptions could have been changed to communicate the DR, but it still wouldn't have hit the other two points.
Gren damage reduction will be communicated/hinted at to the player, and it does serve a purpose considering how relatively easily they are wiped by explosives. Grens are somewhat thematically weak as units, so I wouldn't say DR adds to their identity, but honestly, the balance implication was the biggest reason here.