Mayonnaise is just that, mayonnaise. Miracle Whip is a salad dressing, says so right on the label. And I like both of them depending on what the rest of the recipe calls for. I would think if it called for a salad dressing and didn't mention, Ranch, Italian, French, Russian, etc, they meant Miracle Whip.
I remember my aunt 'making' salad dressing with mayo, but she added mustard, sugar, pickle juice and more. It turned out to taste like Miracle Whip. So maybe when it came on the market, was a short cut for plain mayo.