It's weird to presume that every movie goer would be familar with the events sorrounding Manson and I assume the movie will age worse than Django and IB because of it. Still many standalone scenes were amazing. 
Not really the movie's fault.
You wouldn't complain about Ida needing some historical context to be understood, for example.
That's true for every piece of art, really.
Old art, literature, etc. usually comes with something or someone explaining you the context. Certain historical backdrops for movies don't need further explaining currently - i.e. something like general knowledge of WW2 is basic education. Yet if people didn't follow the marketing of OUATIH and just went to see it, one third of the movie might fly over their head and not because they are too dumb.
I'm not faulting anyone for not knowing about Charles Manson or Sharon Tate, but it's still not the movie's responsibility to take you by the hand through every cultural reference.
Watch the movie, read on the thing, rewatch it again, if anything.
Also the cultural context you mention being basic education varies from to culture to culture, so when you watch foreign movies you miss A LOT of cultural elements that could be assumed to be understood.
You can watch something like Jia Zhangke's Still Life without knowing anything about Three Gorges Dam and all that shit (which in the west is certainly less common knowledge than goddamn Charles Manson
) and miss many aspects of the plot.
Of course with Tarantino, quotationism is integral part of his signature style, so going in you should expect it to be the case.
More generally speaking, i think movies have a problem of explaining themselves too much, if anything, because the audience is expected to watch them once and move on to the next thing, which makes for very dull art all around.
Having to watch a movie more than once to fully grasp it is a feature, not a bug.
