Answer;
This is because the nervous system would no longer be linked to the endocrine system.
The reason as to why the adrenal and thyroid glands atrophy if the pituitary glands cease to function is because the nervous system would no longer be linked to the endocrine system.
Thyrotrophic and adrenocorticotrophic hormones from the anterior pituitary keep the adrenal and thyroid glands in action. If pituitary stops secreting them, it is obvious that adrenal and thyroid would be affected.