I'm not sure how much wealth Doom or Latveria has, but I suspect if Doom really wanted to stockpile adamantium armors, he probably could. He could probably also magically teleport to the Asgardian dwarves and work out a deal to get a few uru metal armors. But let's just chalk this up to editorial or narrative "blindspotting." That's usually the main reason why characters don't do what we'd expect them to do, if we were in their shoes.