'cos canvas is a pain in the situpon to sew curved.
That's true actually
and being tabby woven if frays like blazes if the seams and bindings aren't sound. Denim has flat felled seams to try to stop the worst of the fraying, but try doing that on heavy canvas and curving it.
The caveat is kit bags, but even they often had square bases on them.
I presume there's also the point that the canvas packs generally imitate older portage packs, and the bulk they were able to carry mattered more than shaping the pack, especially if that made packing them awkward.
Basically you can fold a sheet of canvas into a very sound pack around goods, etc., and just add leather strappings to help carry it. The packs sort of copy that.
M