Why do diabetics have to be so careful with their feet?

Thank you all for your explanations! 
Diabetes is the leading cause of nontraumatic lower-limb amputations in the United States [4]. Although it is a chronic and progressive disease, proper management and foot care can prevent or delay a majority of amputations [4]. Blood vessel damage in the feet is a problem for all people with diabetes, but managing the condition well can significantly reduce the risk [2]. However, it is important to keep a careful eye on your feet to allow any potential problems to be addressed before they worsen [2]. Changes in the structure of your foot can affect the normal pressure distribution, leading to high-pressure areas that may develop into ulcers if not treated promptly [3]. The best way to avoid foot ulcers and other complications is to follow your doctor's orders, practice good foot care and maintain a healthy lifestyle [4].
People with diabetes are prone to foot problems because the disease itself can cause several issues. [1] The first is decreased sensation in the feet, which can lead to injury, infection, or ulcers. [1, 2, 3, 4] A second issue is the lack of oxygen delivery to the feet, which can cause tissue damage. [2] Nerve damage, which is common in diabetics, can also lead to loss of sensation and/or circulation problems. [1, 3, 4] Often, these issues will manifest themselves in the form of ulcers and damaged tissue, which can become infected if not treated properly. [1, 2, 3, 4]