Because it is a European institution established by countries including GB after the war to protect it's citizens from the state? It is there, amongst other things, to protect citizens from torture, slavery, gives them a right to a fair trial and right to privacy.
All protections that are holding GB back of course. So let's chuck all that away to enable a venal, dishonest, free speech attacking, and self interested government to gas light it's citizens even more. Let's be like Russia and Belarus, the only countries in Europe not signed up to the convention. One thing that the Boris Johnson saga showed was how much of our constitution is at the whim of government and if the government doesn't respect it then there is little protection for the rest of us.
I have no idea what the Canadians and New Zealanders have in place, (although I am sure that their indigenous populations would love to have recourse to some of the legal protections the convention affords) but given that our freedoms are increasingly under threat from the Tory right I'd say the EHCR is pretty crucial. If it wasn't they would be so bothered about it