Originally Posted by SugaredSaffron
It's not because people are used to using butter, it's a superior product to margarine :/ Margarine is coloured and flavoured to imitate butter. They are usually salted and margarine icings are usually also softer at room temperature. Butter is churned from cream, margarine is made in a freaky lab.
The fact that butter and margarine are processed in different ways does not necessarily make one superior to the other, they both have their place. I'm not sure how the relative freakiness of the manufacturing facility is relevant, or how you would quantify that.
In our experience (and in the experience of our customers) the margarine-based BC recipe we use produced superior results to butter-based BC, but that does not mean margarine is always superior to butter, just that our margarine-based BC recipe was superior to the butter BC recipe (which was still pretty good).
We did not have any issues with temperature sensitivity in our BC until we switched out shortening component from Sweetex to Sweetex Z (which has zero trans fats), so if that is a concern and you cannot use shortening with trans fats you might be better off using butter.