Because the properties that make them perform — like a tent that's both strong and waterproof — come from fusing materials together. A canvas tent base is cotton coated on the inside with PVC, a plastic, and once the plastic is infused into the cotton it is nigh on impossible to separate, and therefore very challenging to recycle.
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Ben explains that a tent has to perform in challenging environments, so it needs to be durable and waterproof. The base is often canvas — cotton, an organic material that's strong but absorbs water — so they coat the inside with PVC, a plastic, to keep water out. "Once it's infused into that cotton, it is nigh on impossible to separate." With single modern materials — only cotton, only PVC, or only polyester — there are ways to recycle them, but once they're fused it becomes incredibly difficult, nigh on impossible to remove, and in reality may be impossible. He notes it's early days, with steps being made to recycle these combination fabrics, including enzymes that eat one material away and a startup he'd heard of in Spain and Portugal developing heat-dissolvable thread that melts in a heating chamber so the panels come out as just cotton.