One can make the case that the NDP, and its predecessor, the CCF, never, not even under Jack Layton in 2011, really aspired to become the government of Canada. It's aim, it's self-appointed role, was twofold: to advance the progressive and socialist causes, especially by pressuring/influencing successive Liberal governments; and to be the conscience of parliament by, generally, advocating for honest and fair dealings and conduct; think about JS Woodsworth, MJ Coldwell, Stanley Knowles, etc.
One can also argue that, under Tommy Douglas, the CCF ran a pretty good, fiscally responsible provincial government in Saskatchewan in the 1940s and '50s.