Because it has Imperial roots which the left LOATHS. If visible, tangible good were to come of our Imperial heritage it would undermine a core tenant of their ideological grounding.

Part of the reason the left is so enticed by the EU (imo) is because they are deeply ashamed of our history and heritage and the EU represents a blank slate. It has no heritage and so can be placed on a pedestal as an unambiguous good, even as the core of the EU structure deeply conflicts with the lefts core traditional beliefs.

ANYTHING associated with Empire is bad. Unambiguously. Not up for debate. Because if everything the nation was is abhorrent, its justifies its complete abolition and replacement with something new. Some cleansed in their eyes and build in the image of their ideology.

CANZUK would represent a return to the old way, a year that began divergence in the 1800s with its final nail in the '70s as the UK joined the common market.