According to the BBC, there are numerous advantages to having an endoskeleton rather than an exoskeleton, including a sturdy framework for the body and the potential for continued growth. Exoskeletons do not grow with the body, significantly limiting the stature of any given creature.
The endoskeleton provides bodily structure, allowing the organs to remain in separate areas of the torso while providing upright structure. The ribs and the cranium shield the heart, lungs and brain while the vertebrae give the human body its upright structure. Because the skeleton also grows as the human body grows, the organs and musculature are allowed to develop fully until the body reaches physical maturity. The State University of New York System notes that endoskeletons are more adaptable to supporting a larger weight since they can do so without becoming too heavy on their own. Endoskeletons also grow smoothly without the going through the molting process of exoskeletons.
Endoskeletons allow for the easy attachment of muscles and flexibility. They have numerous joints, which give the body a far greater range of motion than it would have with an exoskeleton. Likewise, the muscles attach directly to the skeleton, allowing for far more precise movements and fine motor skills.