Because homeopathy drugs were sold as "cure everything". People were taking them thinking they can treat serious diseases, which they can't, so people were losing time and money when conventional medicine could actually provenly cure them.
Placebo-effect can only take you so far. It can help with everything that is perceptional: pain, depression, maybe minor inflammation etc. But it won't help to treat diseases that root in physical conditions like bacteria, tumors etc. etc.
However shammy people were making placebo for everything. Common cold, liver disease, cancer, you name it. This is highly detrimental for people and society health.
It is especially so for scammers usually bundle in some horse shit with your "medicine". Like conspirology theories (government does want you to know this ancient medicine secret...) or even religious gibberish.
So, that's why. There is a huge difference in consequences between believing in nonsense and using irrational mind effect.