The Russo's have already said why hulk wouldn't come out in IW.
According to the Russos, though, that's not how they saw it.
"A lot of people had interpreted that the Hulk was scared of Thanos. The Hulk has had his ass kicked before, and so it wasn’t like the minute he gets his ass kicked he’s going to run and hide. It’s really the notion that the Hulk and Banner have a very dysfunctional relationship, which was explored even further in Thor: Ragnarok. And I think that relationship is starting to strain between the two of them and the Hulk is not interested in playing the hero to Banner anymore," Joe Russo said. "I don’t know if the Hulk sees the value in the relationship. They both want to control Hulk’s body. The Hulk brings fighting and strength to the table and he’s not quite sure what Banner is bringing yet. So, I think this is becoming a stubborn push and pull between two very despaired personalities stuck in one body."
https://www.syfy.com/syfywire/infini...idnt-see-happy