My sense is that as the field gets more cutthroat, established artists are just less likely to give away their own know-how to young competition. So it becomes a vicious cycle of everyone fighting for the scraps they have (much to the financial benefit of publishers).
This is the opposite of my experience in the business. The only people who’re getting more closed-mouthed are the known douchebags. Who weren’t helping anybody out anyway.
One obstacle is that the how-tos of comics are constantly changing. Some specific advice that would’ve been practical 10 years ago is near-useless now. And we don’t have -a- comics industry anymore. We have five or six largely unrelated markets.
If what you mean is their know-how on how to attract an editor's attention and succeed with an audience, the truth is they mostly have no idea themselves how come they made it and the next person didn't.
Never mind how, like Steve says, the circumstances now are very different from when most began.