So where did 2 pounds come from? I suspect it was mostly a behavioral/reality issue. Assuming a value of 3,500 calories per pound fat loss (and I’d note that this value has some issues inherent to it discussed in The Energy Balance Equation), to lose even 2 pounds per week means a net deficit of 7,000 calories per week or 1,000 calories per day.
Which means either a lot of activity (a minimum of 2 hours for someone relatively untrained) or fairly large-scale caloric restriction. Now, certainly this can be achieved with extreme diets but my guess is that the practicality of the 2 lb/week rule came out of this idea: for most people, a larger daily deficit isn’t realistic. At the time it was also assumed that faster weight loss was associated with poorer long-term results but, as discussed in Is Rapid Fat Loss Right For You?, depending on the specifics, the truth is actually exactly the opposite of that idea.
Of course, weight loss isn’t fat loss and the issue of muscle loss is a concern, I discuss this in detail in What Does Body Composition Mean? as well as elsewhere on the site. Another issue regarding the 2 pound per week rule is muscle loss while dieting; this is (or at least should be) a concern and, certainly early studies found that bigger deficits caused greater muscle/lean body mass lost. But there were at least two problems with this: