Simple - one must strive not simply to be A shit but to be THE shit - The plutonic ideal of shit, the perfect shit from which all other shits are derivitive. Anything less is a failure. So following this logic.
“You ain’t shit” = You are invalid from the rubric, so below par as not to be mentionable.
“You are shit” = Acknowledgement that you are shit of average or middling status but with the implications that vast improvement is nessisary because you are still a failure.
“You are not the shit” = More directed pointed reminder that you are far below the goal of being THE shit and maybe are overestimating yourself.
The national identity of being Canadian over the last while has been every four years wondering if the downstairs neighbors are going to decide to restart up the meth lab they keep tinkering with and bracing for the sounds of domestic violence to resume.