Why did the Plantation owners Not want their slaves to learn read?

Because at that time it was believed that literacy was a threat to the institution of slavery. Because first, literacy facilitated knowledge about the successful slave revolution in Haiti of 1791-1804, the end of slavery in the British Empire in 1833, and the writings of abolitionists. And secondly, literacy allowed or potentially allowed slaves better access to information about the Underground Railroad and other routes to freedom.