Why do you think most slaves were unable to read or write?

It was illegal to teach a slave or a "free person of color" to read and write. Anyone caught doing so was often ordered by the court to be whipped in public. Slave owners believed if a slave was taught to read and write they would no longer listen and obey their master and would then be worthless.

More:There was an additional, ultimately far longer lasting reason for keeping African-Americans from learning to read or write. That reason was to prevent their prerogatives under the 14th Amendment of the US Constitution. Section 2 thereof ought to have effectively invalidated the ruling in the Dred Scott case, which counted persons of color as only 3/5 of a whole. Alas, two powerful parties ensured that Dred Scott remained in effect, and that is to put it kindly.

Firstly, the Radical Republican Congress of Reconstruction feared that increased representation gained by the South by the colored vote would weaken their own political power. Ironically, Southerners ensured their own under representation by presenting various barriers to the voting of former slaves, and later, the great-grandchildren of former slaves. One of the essential elements of this strategy was the literacy test. It was not until the Voting Rights Act of 1965 that Federal authorities began to proactively intervene to ensure the rights of African-Americans to vote.

Another view:I must plead guilty to expanding the scope of the question somewhat. I would suggest, however, that in doing so I introduced no issue, which is not germane to the discussion. Besides, history clearly shows that slaves were not quite as ignorant as their "owners" had believed. Witness the many protest songs, which arose from slave culture, notably "Go Down Moses." It appears that while slaves may have lacked literacy, whites failed miserably in their interpretative skills. After all, what else could the line "let my people go" mean, when sung by a bunch of inferior beings? What a failure of imagination to understand that allegory can power an entire race of people, and leave a rich legacy to fly in the face of ignorant slave owners/racists!
They were not "unable" in the sense that they lacked the ability to read. In most places it was illegal to teach them to read. A literate slave might get ideas from things he read, about things like freedom, or how to get away, and where to go if he did. An ignorant slave is much more controllable.

It was even considered inadvisable to let domestic servants learn to read, for the same reasons.
Slaves can't read because as people kept slaves, they did not get the chance to discover how to read.
nobody would teach them... also it would make them as smart as the slave owners and they could revolt
because if the slaves were smarter than there owner then

the slaves could acomplish a lot of stuff and the owners would be useless
because if the slaves were smarter than there owner then

the slaves could acomplish a lot of stuff and the owners would be useless