I just think the writers don't want to make IM and Doom like super powerful. Then they just be like ultron or something.. there isnt a good reason for them to not have it imo.
Kryptonite has been around since the late 40's it was pink when they started. Adamantine was the ancient word for diamond but I first remember it from a Barry Windsor Smith version of the Avengers
How rare COULD Kryptonite be?? I estimate the DC universe started out with several thousand earth masses of the green glowing stuff. It is the radioactive remains of the Mantle of a giant planet. Course Supes did gather up all the pieces and remake Krypton just to keep beings living inside Krypton's sun alive once and a runaway experiment changed all the Earthly Green k into K-Iron. (H'mmm, K-Iron Man)
Adamantine is a binary metal created from two alloys. It has a major weakness in that it can be altered by thought and a shaping device that Ultron 5 (but not his brother Brainiac 5) had implanted inside himself.
Wolverine Vs. Untron 5 turning into Rubbery-Canuck verses Bot-Boy.
I'm new here so I hope this is an adequate response.