Here are several more reasons that are just as important as the only reason you could come up with: distance between patient’s home and trial’s location, health state of patient, trial inclusion and exclusion criteria, timing of the patient’s diagnosis/availability and the trial study period, sponsor’s willingness to pay costs of the trial drug/treatment, wealth of the patient, unwillingness of the patient’s insurance to cover any of the costs, travel, and lodging.

Beyond all these factors add in the type and aggressiveness of the disease of interest and you might also find the study period does not last long enough to sign up enough patients for a rare disease, or the study period doesn’t last long enough to determine survival statistics.