Many commercial cake mixes use vegetable oil, but I have seen some (usually 10% or less of what the store carries) that are "Butter Recipe".
These use butter instead of vegetable oil. What is the difference in these? And is the reason you do not find many in stores is because it is not popular, or costs more, or something else?
Butter gives more flavor, but oil provides longevity.
Butter is about 20% water. That water will evaporate during baking and continue to do so even after they are out of the oven causing your cakes to stale quicker. Also the the fat in butter remains solid at room temperature. Once your cake has cooled the fat, now dispersed in your cake solidifies once again and makes for a bit firmer of a crumb.
Vegetable oil, however, is pure fat containing no water and remains liquid at room temperature. The moisture provided by oil will not evaporate during cooking and even after the product is cooled to room temp, the liquid fat remain and provide a moist crumb. Since there is no water to evaporate this product will last much longer on the shelf than one made with butter.
In addition to Rhapsody's answer, I want to say one more thing. Mixes, by their nature, are designed for cooks of little or no experience. Certainly more experienced cooks use them too, but Duncan Hines wants their cake to to turn out fine even if it's made by a 12 year old.
Butter (see creaming method here and here) is a bit more challenging, oil is more foolproof.
There can also be nutritional/dietary considerations involved in choosing one or the other (saturated vs unsaturated fats, dairy or nondairy ingredients). Also, oil is easier to accurately measure by volume than solid fats like butter.
