There are a few good reasons. It is in Facebook's best interests to keep the conversation positive. If Facebook were to introduce a dislike button, the conversation could quickly turn to an argument, a de-friend, or if extreme enough, leaving Facebook all together. Social media is about positivity and making connections. Disliking something ushers in a great potential to interrupt that utopia Facebook has attempted to establish. Additionally, Facebook is all about the ad revenue. It isn't very smart for Facebook to allow people to dislike their sponsors. Additionally, all things "liked" are tallied and stored so Facebook gets more and more information on your behavior and interests.
But the dislike button needn't be all negative. Users could just as easy dislike a post that has a negative connotation in the first place like "I just got fired." It also leads to some funny dialogue between people. See Youtube's voting system. Various memes have cropped up stemming from the negative reviews of videos. Most notably, often seen in the "Top Comments" section is the "(blank) [number of negative reviews] people haven't (blank) [positive situation or action related to the video]."
So we must ask, if Facebook were to introduce a potentially negative commenting option for people, would you use it? Or do you like to stick with the positivity when it comes to your internet interactions when it is directly connected to your name?