All we hear is that this is one of the great Grey Warden secrets. "It must be protected at all costs." As usual, we're most concerned with deceiving our own people. But why hide that the Deep Roads were shaped not only by the dwarves but also by us?
I found records dating back to 1004 TE, the wake of the First Blight. Early Wardens discovered that some darkspawn could think and speak and commanded portions of the horde even after the Archdemon's death. A few could wield magic with the skill of a Tevinter magister, and the Wardens greatly feared them.
—From Janeka's research notes