I'm absolutely not downplaying what happened in Tlatelolco but I'd wager that in addition to the Olympics aspects, there are other things at play: October 1968

I'm absolutely not downplaying what happened in Tlatelolco but I'd wager that in addition to the Olympics aspects, there are other things at play: October 1968 was the height of the Vietnam War and a US Presidential election was less than a month away, US (and much of the world's) news coverage would have been focused on those. The Soviet Union was still the big ugly monster in the room which means most world history classes focused on that period would talk Cold War and detente. On the other hand, June 1989 was a time of relative peace in the western world. The Soviet Union was around but waning as the Berlin Wall would fall just months later.. which makes Tiananmen stand out that much more. I think most importantly is that cable news and the 24 hour news cycle had been born in the interim. It meant that we saw footage from Tiananmen Square practically as it was happening.