If I had to speculate about your question, I would guess that a) there was almost no digital video to matter a few decades ago, b) there was different culture r

If I had to speculate about your question, I would guess that a) there was almost no digital video to matter a few decades ago, b) there was different culture regarding digital media (or maybe I'm misremembering), trailers were more tame and there was less of them overall (I'm not from USA btw), like maybe 1 or 2 before the movie, not 5+. There were much less story based TV shows with contiguous plot, which are all the fad now. Making a trailer for Seinfeld can be done easily. Now making a trailer to say Game of Thrones will most likely involve some major spoiler (or a dozen). So my guess answer is - the social pressure finally rose high enough that there is now critical mass of people who care about spoilers and got spoiled a few times by official media. That's why there are more posts about this. More instances of the issue multiplied by vastly bigger market due to cheap internet and streaming.