because fabric is made of a series of strands of cotton fibers which are dye'd to their intended color. While there are some naturally occurring colored cotton fibers and a few experiments with making naturally occurring high saturation colors like black and blue, at this point they are not market viable. So with canvas micarta you have a series of exposed fabric strand ends exposed to air, caused by the fabric wearing away slower then the pockets of resin surounding them. while they are soaked in hardened resin, if you take a single strand of that hardened string and bend it a lot, the canvas will move while the epoxy cracks and gets knocked away from the cotton fibers. This is one of the properties that makes canvas micarta as amazing as it is, it can flex before complete fracturing into multiple peices. Once you have this exposed, de-resin'd piece of cotton fiber, it is open to the environment as far as bleaching and color washing is concerned. you end up with a bunch of tiny low saturation color spots on an otherwise black(ish) slab of resin soaked canvas.
Black paper is very dense, and the the paper fibers don't really fray or bend like canvas strands do so the edges remain crisply soaked in resin and in their manufactured high color saturation state.