Oh, I agree. I am just speculating that DC never reprinted it because of the strong similarities to Marvel's Thunder god.
This is prior work and presumably has an up to date copyright. Shouldn't be any reason to worry and it's not like Kirby, Lee or anyone at Marvel invented the Norse God, Thor.
I don't think its because of that -- rather I would think that the editors at DC might have thought readers could have been confused to see a Kirby Thor in a DC mag.
During the early 70s, a lot of DC's sci-fi was being reprinted in giant size reprint titles like Strange Adventures and From Beyond the Unknown - and Kirby had just left Marvel in a huff. Maybe Kirby and the DC editors wanted this story buried to avoid any Marvel connection. Ooo, sounds like a conspiracy theory ... or just some good ol' fashion Kentucky windage!
Again, pure speculation.