It's a combination of bad and/or repetitive product and viable viewing options at home causing audiences, especially older ones, to stay home. If the product improves, that will get some fannies back into the seats, but there's going to be a section of viewers -- and seemingly already are -- who just don't care anymore, and who will prefer to wait a few extra weeks for the convenience of staying home, avoiding the crowds, plus the cost of gas and tickets.
Attendance is probably never going to recover to older levels, simply because viewing at home not only yields better convenience but -- more importantly -- superior product. Add in that the home viewing experience matches or in some cases surpasses what you can get in theaters and that choice is even easier to make.
You can't even use the excuse that Hollywood needs to make better movies...even movies as well-reviewed as War For The Planet Of The Apes -- the third entry in a popular and critically-acclaimed franchise -- have underperformed.
I think that ties into the repetitive product angle. People are seemingly just getting tired of "franchises" and "interconnected universes" -- it appeals to the nerd fanbase, but the casual viewer is being turned off.
Sequels in general are all going to suffer declines at some point, they are easy money for the studios, but keep going back to the same well and you'll end up with SPIDER-MAN HOMECOMING -- which will sell the least amount of tickets of all those films (despite doing "well," it hasn't been this end-all, be-all smash that was anticipated). Yet should it be a surprise? It's the SIXTH Spider-Man movie in the last, what, 15-16 years? Been there, done that, even if it's an "improvement" on the Andrew Garfield movies.
WAR is still a sequel too -- third one in on an old franchise that people are very familiar with. I said from the very start that WAR was going to have a problem because it looked so similar to the last one, and audiences in general seemed to share my lack of enthusiasm. I did see the movie last week, and I liked it, but I didn't love it.
Technically it was fine but I just didn't care for the overall direction they went with this series. Instead of making PLANET OF THE APES, they essentially built an entire trilogy around remaking (conceptually from a broad angle, anyway) CONQUEST OF THE PLANET OF THE APES, which was the darkest, most "realistic" and also most depressing film of the old series.
Not really my bag, and I think they ended up painting themselves into a corner with the 3rd entry. I saw it coming after DAWN and said they needed to advance the story further -- and they didn't do it. Big mistake IMO.