I believe the worst cases specifically are due to systemic organ inflammation, caused by the virus triggering a cytokine storm. Folks immune systems themselves cause disproportionate damage, and the lungs are perhaps the most sensitive to inflammation in terms of being able to function. Ironically, the effective treatment for it is immunosuppressants, and most doctors seemingly figured that out pretty early on. Of course, too much immune system suppression would make the actual virus symptoms worse, and open you up to severe bacterial infection.