Probably concern that they’d look like copycats. Which is ironic, considering how much Image Expo bites off the Apple event format.
I think we’ll see more moves in this direction, though; Valiant’s Periscope’d #valiantsummit before Comic-Con being the most obvious move in that direction, but even Marvel’s All-New All-Different Marvel Previews book was an attempt to control the message in an Expo-esque manner, to some degree.