The lack of alternatives:
Why was there a lack of alternative scientific evidence to support any other theories of what caused disease?
What were physicians allowed to do very occasionally? What happened during a vivisection? What did this mean about Galen's ideas and why?
There was a lack of scientific evidence to support any other kind of theories of the causes of disease. Dissections were mostly illegal, because the Church taught that the body needed to be buried whole in order for the soul to go to heaven.
Very occasionally, physicians were able to dissect executed criminals, or criminals who had been sentenced to death by vivisection.*
When this happened, the physician would sit far away from the body reading from the works of Galen, while the actual cutting and examining was done by a barber surgeon.* This meant that Galen's ideas were preserved: anything in the body that didn't agree with Galen's writings could be explained away, since the body was that of a criminal and therefore imperfect, and the physician himself never did any of the examinations.