It's just processed differently. Steel-cut oats are cut a few times from the whole groat, whereas regular oatmeal goes through a process of steaming and toasting. Steel-cut oats are higher in nutritional value and have a much bolder taste.
Steel cut take a LONG time to cook - and when they're done, they're no better than just plain old rolled oats.
The one is less refined that the other. Avoid refined foods. Go for the Steel-cut.
I actually think the steel cut taste better