Why? They're right across the river from one another, it's just a matter of urban planning which side will dominate, and for Canada having a river between you and the Americans is always a plus.
It isn't so much as issue of urban planning so much as politics. Upper Canada consolidated all of its politics and economy in York/Toronto, to the point that London and Kingston had a smaller growth rate for so being so far from the hub (London also had the additional burden of poor roads and not being a port). if Canada controls Michigan, the center of commerce for the new province becomes Detroit which probably looks a lot like York, which means that just being so close to a major center of influence and wealth Windsor gets a lot more growth (this probably has knock off effects on London too because it splits the difference between the two). In OTL Windsor was the edge of Canada, which means that everything across the border belongs only to Detroit, but a world where someone controls both cities means that Detroit can act as a hub for the western end of Upper Canada which is a big boon for Windsor.