Because of negativity bias.
If you say you had a negative experience with a source, people will take that as overwhelming weight against the source. You can read any steroid thread where people post their negative experience with an injectable.
If you say that you had a positive experience, people will wait for blood test results.
Negative experiences are no more proof than positive ones. You shouldn't trust someone's negative review *or* positive review. Bad reviews could be because someone has a personal bias against a source, or they're a competitor posting under a sockpuppet, or whatever.
I don't have any particular source in mind when I say this, by the way. In fact, after lurking here for a few months, there are very few sources I *do* trust, if any.