[sandworm101]

Because the default with chronically ill patients is no. There are also financial issues. If a patient is chronic then they are likely on ongoing treatments of some sort. If complication arise from the investigational medication then questions come up as to who will pay for what treatment. What if the complications lead to the patient having to abandon the ongoing chronic treatment? So the docs will lean towards no. And in the US there is the huge question of whether one's insurance even allows for such participation, but that is a question for one's lawyer rather than doctor. All such concerns can lead to the exclusion of certain populations, possibly the populations more relevant to the study.