It's not about creating a unique armor that has DR, it's applying DR across the board to existing armor types.How is creating an armor with DR, or a weapon with benefits versus certain kinds of armor types, any different from creating a new magic item?
The module can become accessible in this way.
Which then causes it's own problems. If (making up numbers) you say light armor has DR 2, medium armor has DR 4, heavy armor has DR 6, how does that affect monsters that have a lot of attacks that do little damage? How does it affect monks and barbarians or dex based builds? Do monsters that are not explicitly wearing a specific type of armor get DR? What if the monster is described as being difficult to hit because it's so fast vs a thick and tough hide?
It's easy to say these things (and the other suggestions) can be "easily" added but taking into account all the ripple effects is a lot more difficult.