Red skin potatoes contain a chemical compound called anthocyanin. There is not a lot of this compound in red potatoes. When they are boiled, skipping all the chemistry details, the compound decomposes. When it decomposes, the red is gone. When steaming, I would have thought that it would have had the same effect as boiling. My assumption in English is that the steaming doesn't break down all of the anthocyanin or at least, not as much as boiling does, likely due to the fact that boiling occurs when potatoes are immersed in water and steaming doesn't. When it's in water, the anthocyanin is broken down more.
You may care to note that anthocyanin ranges from blue to red depending on the pH. Thus, this may or may not work, so try it maybe on 1 or 1/2 a potatoe. Take it and give it a brief soaking in citric acid (lime/lemon juice). Then, proceed to steam it. I'm not sure on the exact pHs that will make the anthocyanin turn from red to blue.