They got cold feet on Ava and knew she was in over her head...
Obviously it's impossible for me to know, but I'd say that it was less getting cold feet, and more them not wanting to set her up for failure, or have her try to keep her schedule open for them.
Because if we're being honest, them deciding to give in on the Snyder Cut has put blood in the water on them being willing to give in to fan reactions and input, even though I think it was more a product of right place right time with WB being willing to pump money into content for HBO Max and the pandemic making it a relatively low risk investment. There's a pretty decent chance that if they rolled this out in the next year or two, it would've gotten eaten up by those fans. So, instead of pushing it back long enough to separate it from the last Darkseid appearance, maybe it made more sense for them to bush it for now.