Possible reasons why the nurses and doctors not let my dad eat and drink in the hospital?

NPO: usually, if there is a possibility that he needs a procedure (i.e. surgery) done in the next 8-12 hours, we'd starve our patients, to minimize the risk of aspiration. there are also situations where resting your bowels is therapeutic (for example bowel obstruction, pancreatitis)

Many reasons: He may be having a test or a procedure or surgery and cannot eat or drink before them. Or there may be a GI problem or another medical problem where it would be detrimental to his health to eat or drink. Ask his doctor or the nurse why he cannot eat or drink.