Why were slaves not taught to read?

The ability to read is a degree of empowerment. If slaves could read, they could educate themselves, pass messages, and it was also time consuming. Time spent reading was not time spent working, and an educated man/woman is not as easily oppressed as one with no education.
Knowledge is power. By keeping a population ignorant, they are easier to control. It also prevented them from sending messages back and forth to each other.
The slaves were only considered laborers to their owners and had no need to read.