We have only known these things for the last 5, maybe 10 years. For a couple decades before that it probably should have been obvious, but it definitely wasn't known. And of course before that, it was just getting your bell rung, nothing more, nothing less.
It's worth noting that the severity of concussions way back then was probably a lot less despite (or perhaps due in part to the respect resulting from) lack of helmets due to the speed of the game and less physicality.
Remember when concussions killed Lindros' career? IIRC it was mostly regarded as a shame that he couldn't be a good hockey player anymore...it probably isn't a good idea to keep getting hit in the head so it's probably for the best. (starting to have an idea, but far from knowing or fully understanding the impacts of impacts)
Think about Crosby a few years ago. That was the awakening for me, as well as many others I'm sure (no doubt these reports existed before, but certainly not as widely known). The guy can't get out of bed? A sliver of light through the curtains feels like a lightning bolt to his head? Vomiting? I'm of course paraphrasing, but the repercussions finally hit home and felt much more real. The media environment of today played a major role in this.
Having said all of that, I don't think the NHL knew any of these facts before anyone else. Perhaps they should have, but no one was connecting the dots yet, not even medical academia (and if they were, they either did a poor job of sharing the info, or didn't even try to for proprietary reasons). If anyone can provide evidence of the NHL being intentionally negligent or malicious then I'll change my tune, but this is a problem with no one to really blame.
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