The reason why people with Down Syndrome can't speak clearly is because people with Down Syndrome often have an unusually large tongue and smaller upper jaw which makes pronouncing words more difficult.
People with Down Syndrome may also have speech difficulties as a result of cognitive delays, physical differences in oral structures and functions and even difficulties with learning and memory.
These things can also impact a persons speech development and coordination as well.
People that have Down Syndrome often have a small oral cavity but a relatively large tongue and a narrow high arched palate which can also affect the production of speech.
People with Down Syndrome may also have low muscle tone in their face that makes it harder for them to coordinate the needed muscles for speech.
And some people that have Down Syndrome also have hearing difficulties that can also further impact their speech development.
Cognitive differences such as attention span and memory also play a role in the speech of a person with Down Syndrome.
Challenges with attention span and verbal memory can impact speech development and people with Down Syndrome may also have slower learning capacities which can also affect the persons ability to master speech and language skills.
Motor speech difficulties and language development also play a role in a person with Down Syndromes ability to speak correctly.
Some people with Down Syndrome experience dysarthria which is a motor speech disorder which affects the muscles that are involved in production of speech and that can lead to reduced speed, range of motion and coordination of the articulators.
Apraxia can also play a role in speech as symptoms of childhood apraxia of speech which is a neurological speech disorder have also been reported in people with Down Syndrome.
Language development is also often delayed in those with Down Syndrome and leads to later first words and slow vocabulary growth.
People with Down Syndrome may also have difficulty in mastering complex grammatical rules, which lead to the use of short, telegraphic utterances "keywords without function words" and they may also have difficulty in pronouncing words clearly.