I didn't think of that, actually.
See, from what I know about Darkseid, this misty form is probably because something knocked his power down several levels, and he's stuck in the dimensional barrier between Apokalpis, and Earth.
Waiting and growing in strength, until he can materialize as you said.
Perhaps he tried to fight Nekron? "DIE!"
(Nekron is the aspect of God's Death, or rather the ability to kill people by looking at them and saying die. Or touching them. He also created the Black Lanterns.)
Cause, uh, in the comics, Darkseid could show up on Earth any time he wanted to or just about. I mean, he worked at McDonalds one time, I don't think they have any on Apokalpis.

I mean, when he wasn't dealing with stuff on his home planet, he would go to Earth a lot.
And yes, I can't spell that name to save my life.
Also, Darkseid is a God, and one thing he suffers from is his power is given mostly by his followers. Kill all his followers dead and he'd rapidly lose his strength/invulnerability to the point where I could kill him.
The Our Worlds at War saga was interesting, because Wonder Woman needed his help, and his powers were so weakened, that she had to get more people to pray to him.

To power him up again.