There are a number of reasons for this.
First, there is the obvious reason that a natural illness cannot be deterred by threats. Humans might well hesitate to release a truly virulent bioweapon for fear of what the consequences would be. Diseases do not do this.
Second, a human-created bioweapon is more likely to have an antidote or some way of fighting it. It is at least possible that scientists would have developed such an antidote along with the weapon.
By contrast, natural diseases can be difficult to prevent or cure. Just two examples of this are the fact that we still cannot prevent or cure AIDS and that we have a limited capacity to fight influenza. It is also true that years of using and misusing antibiotics has helped in the creation of pathogens that are resistant to such drugs. If a new disease were to arise that was both highly lethal and highly communicable, the scientific community might well struggle to get it under control in time.
For these reasons, a natural disease is both more likely than a human biowarfare attack and more likely to be impossible to combat than such an attack.