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In a smart story on ScreenCrush in June, writer Matt Singer discussed some of the reasons this may be the case. In short: after a brief pinnacle where studios and fans enjoyed a happy medium of access and promotion, the crowds grew too big, more and more companies tried to establish a presence there and the cost of rising above the noise and getting people’s attention became too great.
That Disney would not be attending to offer anything new related to its Marvel Cinematic Universe films certainly got a lot headlines, and deservedly so. The Avengers movies in particular have a history of using Comic-Con to get fans excited, introducing the full cast of the original movie in 2010 and then bringing out new additions to the heroic or villainous lineup in 2014 for Age of Ultron and 2017 for Infinity War.
From The Hollywood Reporter: