It's always good to maintain a variety in your diet, and choose foods from many sources.

I think, perhaps, people only thought "beef" when they thought protein (or meat), and forget that there are many choices to be made. If you look at any of the list of "acceptable" foods from Atkins, you'll see lots, besides beef.

However, it seems like people always need a "new" angle on things. As someone else said, it's possibly marketing - make Atkins seem "new" again.