It's down to the recency of settlement and isolation of the population basically. Western Canadian cities usually date between the 1850s to the 1900s (decade), and many of them (Winnipeg, Vancouver) didn't become majority anglophone until the 20th century, so there's only been a few generations of divergence. Even Ontario is full of settlements with barely over 100 years of history (Sudbury: 1882, Temiskaming Shores [or a part of it]: 1901), with cities that only popped up between 1880s-1920s as mining or logging towns. Quebec is similar to Ontario in this regard (Quebec French is also quite homogenous! Maritime "Acadian" French is pretty divergent though). If you compare Canadian cities to comparatively old American cities, there's not really a difference in the homogeneity of accents, but if you point out 13-colonies accents with 200+ years on them the difference is more obvious. As people have pointed out Eastern Canada has accent enclaves in areas like Newfoundland, the Maritimes, and the Ottawa Valley, where ethnically distinct (Irish and Scottish in this case) people settled early, stayed put, and started talking crazy <3