Parallelism.
In my carefully-forgotten past on the old board is a timeline with a POD around 1900 where I planned a Communist Spain that included Portugal and Morrocco. Oh, and pseudo-Nazi Britain was a unitary state with Australia, Canada, New Zealand, India, and South Africa. Why not just leave it at Spain and Britain, and explore the resulting world? Without the political unions, neither would have been strong enough to play vaguely the role their OTL equivalent did. Which is to say, no reason in particular.
Amateurs tend to default to matching identical circumstances to different places. Some never grow out of it (see Turtledove) and some use it and do it justice (see Into This Abyss).
When splitting up America, the automatic knee jerk is to parallel either ancient Rome, modern Europe, or (very occasionally) the Soviet Union. Modern Europe is the usual choice, but there are too few states and large oppressed minorities in North America. So, easy, just make more of both!