I think its because not everyone has gotten the game off of steam, (Humble Bumble exists) And because of the, community wise, simplicity of a website. I believe.
Steam workshop is really terrible, horrible, worthless piece of junk, never ever even consider something of such. Best way is to download .zip of the mod and check what is in it, then if everything looks okay drag and drop where it belongs, that is fast and easy, but especially safe. Okay, I have problem with automatic systems in computers, always prefer manual way.