I think cotton is hard to spin both because of the short fiber length and the structure of the fiber. I found this picture(microscopic photo) and while I cannot vouch for its accuracy to would explain some of this. The far left of the photo shows the wrinkled like appearance of wool. It makes sense that it would grab onto other fibers more easily. Notice all of the first four fibers have these same wrinkles. The last four strands do not. These are considered harder to spin. What helps silk out is its long staple length. I have not tried to spin linen yet. This photo over simplifies things a lot... I would imagine a lot of variety in the different wool breeds and also in the different types of cotton. I would love to have this type of microscope to study these things
Recently I watched another spinner spin some Corriedale that was some of the softest I had felt. I would love to look at this fiber under a microscope and compare it to the Corriedale I regularly spin and see the differences. I would also love to do this with the two cottons I have been working with. I think the science of fiber is fascinating.