I suspect that AC views the A220-100 as having too much range for the missions that AC would deploy them on. Why does that matter? All the extra structure and engineering that went into the aircraft to support it's range is weight that AC would be carrying every flight despite not coming anywhere near the maximum range.

To put it simply the A220-300 has filled it's role exceptionally well as AC's primary transcon aircraft. The rest of the narrow bodies (319/320321/737's) in the fleet provide additional capacity for higher demand routes. The A221's range of 3600NM is much to far for AC regional operations where flights rarely surpass, I'm guessing, 500 NM (most flights from smaller communities simply fly one of the nearest hub airports, yyz, yul, yvr, yyc). And the capacity of 130 pax may be to high for some of AC's smaller routes where 60-80 seat aircraft are better suited.

That being said. I'll throw out my pipe dream again. What if AB were to shrink the 221 further. Removing 20-25 seats to a capacity of 80-100, and somehow reengineer the fuel tanks to bring range down to 1000'ish NM. Then maybe AC would be interested. Call it the 220-50 or 219. This plus a 225 stretch would make the 220 family an incredible force across multiple mission types.