My personal opinion is that cannabis has been illegal for so long (and still is in many places) and thus prevented proper controlled studies and scaled agriculture experience (but the good news is that some of that does exist now if you hunt for it). So instead people relied on personal or group experimentation that was passed down or shared in various forms (it's actually fascinating to think about as it is one of the oldest crops for mankind, predating even corn, and we are using some form of this passed down to us). Some of this cultural knowledge works well and some doesn't. And to further muddy the waters, plants are pretty adaptable, and different environments or climate patterns can make those methods more or less successful.
As a new grower, I found it hard that even in the grow bibles or online manuals, there was no clear prescription of the easiest "go to method." Much of the knowledge contained can be wishy-washy, suggestioning you can try one way or another, and sometimes even contradictory. And yes, many are produced with the goal of selling something, and sometimes that impacts what they write--but often the lack of scientific proof that one method is better than another impacts it more. But this is really no different from gardening at large, there's lots of misinformation in the void of scientific proof and plenty of companies that promise their products will work (and some do, but not all, and not all to the same degree).
One source I have found solid information to dismistify fact from fiction is a scientist gardener Robert Pavlis who runs a blog called Garden Myths (also has videos and books). He doesn't focus on cannabis, but rather how to grow just about any plant well. I like him because he is always studying and experimenting, and when he figures out he was wrong (based on new scientific studies), he'll admit it. I don't think he's perfect, but probably the best overall core knowledge I have found.
I also try to look at cannabis controlled studies, PubMed has a bunch. Some are very objective and provide clear guidance. Others are covertly industry sponsored or provide unclear results, so you have to read carefully.
This is also a crop that too me is a bit like growing the biggest pumpkin for the county fair, most people don't want a wimpy yield. Therefore, there are lots of people experimenting with techniques that push the boundaries. Part of what I like about Grow Diaries is that you get to witness all kind of technique, experience, and experimentation in action.
But great question, I'm definitely eager to see everyone's responses!