Canon could have easily churned out a mirrorless with a 5d mark ii-like sensor in it with a similar body style to the EOS-M, and kept it priced competitively.
Pricing a camera with a FF sensor competitively is pretty hard. Just look at the fixed lens Sony RX1 as a guide. Want a cheaper FF camera? You've already got the D600 and 6D.
Canon is in the business of money making. I'm sure their marketing staff must have done their homework and concluded APS-C is the way of the future. The sensors are cheaper to produce and the accompanying lenses are also smaller. Until manufacturers find a cheaper means to produce FF sensors, they'll always be reserved for a niche market.