The reasons for delayed development in kittens include the following:

Genetic factors: If a kitten has a congenital genetic defect, it may experience slow or stopped development.
Malnutrition: If a kitten's daily diet is monotonous, or if the owner doesn't provide enough food and water, or if the kitten is a picky eater, it will not be able to get enough nutrition, leading to emaciation and stunted growth. If a kitten's nutritional needs are not met for a long period, it can easily develop anemia, hypoglycemia, and other diseases.
Causes of illness: Intestinal parasites such as roundworms and tapeworms can weaken kittens, thus hindering their growth. Endocrine system abnormalities, such as hypothyroidism, can also impede kitten development.