Not to discourage discussion, but you should check out this excellent answer from /u/pettylarceny to a similar question.

There are a number factors: doctors did know (by modern standards) what they were doing in some cases, the placebo effect is a very real and potent thing, when in a bad spot most people would rather try something than nothing (there are also an awful lot of really gruesome illnesses and injuries), if there's prevalent cultural belief that something works you're likely to go with it, etc.