There are many reasons:
The system has locked in inefficiencies, particularly inefficiencies of scale.
It has rewarded a tiny percentage of farmers at the expense of all other farmers. Especially those who inherited quota are able to outbid neighbours for land, machinery, technology and other services.
There are some quota holders who have become multi-millionaires while neighbours struggle to survive. Joe Hudson of Burnbrae Holdings is an egregious example. He fought against supply management for the egg industry, but later came to embrace it and become the largest egg quota holder in Canada and the largest egg grader and egg processor. Burnbrae Holdings is worth hundreds of millions of dollars in quota alone. This is not what any of the original advocates of supply management envisioned or would condone.
It has become an almost exclusive club; few are able to buy their way into marketing board membership. Most inherit the privilege.
All of Canada's lowest-income people are forced to pay more for supply-managed commodities to sustain quota-holders' privileges. It is an entirely regressive tax.
To add insult to injury, quota holders are now being compensated billions of dollars for minor "losses" related to international trade agreements with the United States, the European Union and Pacific Rim countries. Meanwhile other Canadian farmers who are suffering because of international trade and unfair competition are getting little or nothing.
Farmers and politicians argue that supply management means subsidies are not necessary. This is a lie. The system itself is the biggest subsidy in Canadian farming history. The economy and the public would be better served by subsidies than continuation of supply management.
To be sure, agriculture is far from the only sector of the Canadian economy that is bedevilled by supply management. Telecommunications is another. But those other sectors are open to disruptive innovations, such as Uber breaking into the taxi market.
It's a good time to re-examine whether supply management serves the public and farmers are intended.