The reason that canvas tents like the Springbar Highline are so expensive is because they are made from natural materials. In the case of the Springbar Highline, the company has to source natural cotton and then weave it tightly into a dense material. That is what provides the unmatched weatherproof qualities, but it also makes it thicker and much heavier.
Synthetic fabrics are usually used in tents because they are so much more affordable to produce, cut, and sew. They have the added benefit of being significantly lighter, which tends to be a big thing for campers, backpackers, and hunters who have to carry the tent far out into the wilderness.