In The Walrus, Robert Fulford identifies precisely when Canadian cabinets were neutered:
Over lunch one day in retirement, Lester B. Pearson looked back on the men who had served in his cabinet and quoted Napoleon’s remark that “every French soldier carries a marshal’s baton in his knapsack.”
Pearson wasn’t comparing himself to Napoleon. He was talking about ambition. Just as Napoleon’s troops dreamed of high command, many of Pearson’s ministers saw themselves as future prime ministers. And sure enough, when Pearson retired eight of his ministers announced they would run for Liberal leader—each with his own dedicated following and distinct point of view. One of them was Pierre Trudeau.
No one ever heard Trudeau express nostalgia for the Pearson years. In fact, he seems to have hated every minute of it. He saw no reason for ministers to establish their independence by leaking dissenting opinions to favoured journalists and constituents back home. Such freedom, which Pearson had put up with, didn’t strike Trudeau as democracy in action. It seemed more like chaos.
[…]
This truth is best explained by Trudeau’s inclinations, since hardened into custom. In the spring of 1968, as soon as he became prime minister, he tightened the reins of government power and let it be known that those reins all led to the PMO. In the early years, it was said (and widely believed) that his principal secretary, Marc Lalonde, held daily meetings with the executive assistants of all government ministers, so that Trudeau and his aides could know precisely what each was doing. As time went on, they increasingly did Trudeau’s bidding, which remained the case until he retired in 1984.
Since then, with one exception, no star ministers have blossomed under three long-running prime ministers, Brian Mulroney, Jean Chrétien, and Stephen Harper. That one exception is Paul Martin, Chrétien’s finance minister, whose talents attracted constant publicity and many admirers. As everyone knows, the Chrétien-Martin relationship ended in acrimony — the sort of political finale Trudeau carefully avoided.