From Dollinger (2008) A New Dialect Formation in Canada I got it from this old post in r/Linguistics Why are there so few strong regional accents in Canada?

the early Ontario population was characterized as a highly mobile one right from the beginning. As the Canadian west was settled in the 1870s with the newly-built railway lines as its backbone, remarks on the homogeneity in CanE across the continent were found. Lighthall (1889: 581) refers to the no- tion of a "practically uniform dialect" across Canada (if only to deconstruct it from a dialectologist's point of view) and by the beginning of the 20th century the notion of a homogenous CanE was firmly entrenched (Priestley 1968: 76).

It more or less says that the Canadian West-Central accent became so uniform because settlers moved in so quickly and remained mobile and connected with one another.