The second problem is that the story of "Once Upon a Time In Hollywood" doesn't earn the plot twist in the same way that the other two movies do by working so hard. In "Inglourious Basterds," viewers witness the horrors of Hitler's anti-Semitic policies, and see multiple characters attempt to fight back against those horrors before the narrative culminates in the assassinations of Hitler and other Nazi higher-ups. In "Django Unchained," Tarantino shows how slavery destroyed the lives of innocent people and establishes how racism was embedded into the society and culture of the South. When that story climaxes with the ular character obtaining vengeance, even audiences who weren't well versed in the history of this period understand where it was coming from.
"Once Upon a Time In Hollywood," on the other hand, has a more meandering plot. Tate is presented in the film as the embodiment of what Tarantino clearly views as the halcyon days of Hollywood — the carefree, spiritually liberating apex of the 1960s — but the entire story doesn't revolve around either Tate or the Manson family. It is, at its heart, the story of an aging actor who needs to reinvent his career and his close friendship with a stuntman suffering his own subtle existential crisis. Their eventual ability to save Tate isn't the inevitable result of their narratives, but comes out of nowhere, feeling shoehorned in rather than organic.