Because they are unable to hear and learn speech while growing up, due to being deaf. Some learning is possible through lip reading, but can be very difficult.
Even if a person becomes deaf later in life, after having learned to speak correctly, it can be difficult to maintain good speech patterns, tone, and volume levels. Because they are unable to hear the sounds they are making, they can't be sure they are making the right sounds. It would be similar to attempting to write a sentence on a piece of paper without looking at it. Try it sometime, it's frustrating.