In the Guardian
https://www.theguardian.com/film/2020/oct/06/tenet-didnt-just-fail-to-save-...
I haven't seen the film but from the Internet chatter about it, I would broadly agree with this article, given that there were more obviously attractive and familiar "tentpole" films that surely would have drawn more people out (which is what the cinemas wanted/needed). Examples in the article.
"Was the general public simply not ready to go to the movies? Or was Tenet specifically not the film to lure them back?"
... "It’s quite possible, then, that Tenet would have been an overhyped underperformer even in a non-Covid summer movie season"
Let's see. Tenet is a high-concept, big budget, original sci-fi not based on any existing popular source material. The most recent example of something similar, is Jupiter Ascending (Warner Brothers again) whose failure (worldwide $184 million off a $176 million prodution budget not including marketing etc) somewhat stifled the Wachowskis' careers for a while.
(in fairness, Looper did very well from a $30 million budget and for most people including me, not knowing the source material, Edge of Tomorrow was "original" and that made a lot of money, so maybe my point is moot, but the marketing on those two made them look like the fairly watchable things that they turned out to be, whereas the elliptical ambiguous marketing on Tenet just didn't come across as attractive...)