E6 is a highly standardized process, meaning that there is very little you can tweak. As with all color processes, E6 development works at first like B&W development (reducing silver salts to solid silver). However, the byproducts of that development also react with chemicals in the film to create colour dye spots. At the end of the processing, all the solid silver is bleached away, leaving the dyes only. I'm simplifying a lot, and skipped the reversal step, but you get the idea.
So think of it that way: in E6, you reduce silver salts (AgX) to solid silver using only one very particular developer, because crucial subsequent steps depend on it. Silver is only an intermediary product. With B&W, solid silver is your final product, so you can make it any way you like.
E6 restricts variability at the level of AgX -> silver, so the only variable left to control sharpness is the choice of film you make. Think of it that way: if there was only D-76 available for B&W, the only variable left to you for controlling sharpness would be film choice (optics nothwithstanding in both cases, of course).
Another thing you should bear in mind is that the image properties of dye clouds are not the same as the image properties of silver grains. The latter are opaque, while the other ones are translucent. Light scatter and impression of graininess is different because of that. In fact, an underexposed slide (or an overexposed color neg) will have finer grain than an overexposed one, because thicker layers of dye clouds show less visible impression of grain.