PVC is plastic. Plastic has barely been around for more than a hundred years, but metal has been around for millennia. Also it is a lot easier and quicker to slap four rectangular sheets of metal together than it is to craft round tubes, plus many people do find rectangular shapes fit the form of a rectangular building aesthetically, plus it moves water all the same.
Because the metal crimping device they used onsite to construct the gutter made rectangular gutters, because four 90 degree bends were easier to crimp then a perfect circle. Once a downspout was made, they would slide it over a flat long board along the seam and hammer the folded join to hold it in shape, and this is also easier with a flat side.
Construction sites used to have sheet metal workers similar to how they have carpenters, plumbers and electricians. These sheet metal workers would do various bits and pieces of metal sheet on the house where needed. For example joins could be covered with metal sheets to prevent the weather from entering, especially on the roof. They would also be making the gutters and downpipes for the house. They could technically make round ones but it is much harder to work with round tubes then square ones. So most of their work would be with square pieces of metal that they would bend into shape.
But changing labor cost and material cost as well as improved factory lines made pre-fabricated components that anyone could assemble much cheaper then someone custome making things on site. And if nobody needs to make custom changes to the product they can be round which gives some advantages such as easier factory manufacturing and less material for the same cross section. You did also get metal round pre-fabricated downpipes but depending on the local material cost they may not have become common before PVC became cheaper then metal.
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