OP is talking about Canada's supply management system in place in some areas (including eggs). Supply management is a government system that basically puts all large scale production of these items under both a quota system (farmers need to get/buy a license/quota), and fixed pricing to the farmer.

It is a form of farming subsidy that was specifically structured to avoid passing money through the government, and structured to control price volatility.

This is separate from the commercial market concentration in Canada, which is definitely real. To a degree, this is just the reality of being a smaller market. The depth and breathe of the American economy is definitely at play here.

If you compare Canadian super market consolidation vs Germany or UK (for example), you find a much more similar situation.