because remember that the second postulate requires the isolation of the microorganism in order to culture it in an agar medium.
not all pathogens can be isolated. Viruses are non-cellular pathogens- they can only survive and replicate their genome using host cells and not out of it. THis means in order to isolate viral pathogens, you would also need to isolate host cells which is difficult without harming the host in the process
koch's postulates are also limited in establishing causation for diseases like leprosy as well as malaria (coz the pathogen plasmodium grows in the gut of the anopheles mosquito, thus it's difficult to culture it in a laboratory)