Because they had a obtained F101's by then to replace the CF100 in the interceptor role ?I wonder, why did't they buy Phantoms in late 60s/early 70s to replace Canucks?
I suppose in theory they could have obtained F4's in the late 60's or 70's and replaced the F101's, and or F104's with F4's. I'm not sure how long the SAGE capability of the F101 may have been an issue vis a vis potentially replacing the F101.
The "off the shelf" ability of the F101 to use the Genie may also have been a reason to retain the F101. I vaguely recall reading of proposals to equip the F4 with the Genie but don't know what came of those efforts.