It costs more to make things multi-voltage. For small electronics that run off DC, it turns out that that ‘costs more’ is less than the ‘costs more’ of dealing with stocking multiple different power supplies to be sold to different regions.

With higher-power appliances that don’t just run off DC, where you actually need the thing itself to be different, it turns out that having multiple variants (or only selling to one region) is cheaper.

Toasters are a great example because you’re pretty much just shorting a wire into the plug, but it’s a wire whose resistance is very carefully chosen so that it gets hot but doesn’t burn up. To make something like that multi-voltage you would need multiple paths for the power to flow and some sort of ability to switch things, all of which costs more money than not having that.