A bit of history
DC in Canada used to be the main credit card partner of Aeroplan (with CIBC). Those were the days a DC card also meant Maple Leaf Lounge Memerbship and access to the MLL. All points earned by a Canadian CC were automatically transfered into AP at 1:1 ratio. I earned hundreds of thousands of points that way in my AP account!!
About 5 years ago, AMEX signed up a deal with AP for several hundred millions dollars, and obviously, DC was dropped by AP. DC Canada had to align itself with another points scheme program and since there was already a DC points program in the US, it was basically copied by DC Canada, with local adjustments.
Amex requested from AP to cut all ties with DC, including the possibility to transfer points from DC Canada to AP. Otherwise, every cardholder just would have hold on to their DC points and transfer them into AP when needed. Nobody would have signed for the new AMEX card!
That basically explains why you can't transfer from your Canadian DC points into AP.