Because if the only way you can interact with a post is by voting up or voting down, then a few problems occur. First, any instance or app that lets you use the downvotes now has a stronger voice than anyone who isn’t using the downvotes. Second, it means users can not push down bad content without a consistent supply of new content to vote over it, a massive problem for small communities. And third, it fragments the community making the service harder to use.