Why? Apart from the particular medical benefit of each preparation, the common features of bitterness and hot serve good purpose. And the purpose is: to generate chi. And chi is an important "broad-spectrum" healing-aid. Bitterness can generate chi, and so can hot drinks. And the result is more noticeable when a person is in a relaxed state, which is normally the case for a weak patient!
Corollary 1: Food with bitter taste likewise can help generate chi (e.g. bitter gourd). The drinking of hot water can generate chi, no wonder Chinese doctors generally advice against cold drinks, and no wonder Chinese like drinking hot tea.
Corollary 2: Chinese medicines prepared in the form of pills, though more convenient, will lose part of their healing ability, sometimes even a major part!