Whenever the topic of Maslow's hierarchy is broached, a number of questions are always raised. The one that you have asked here is a common one.
Essentially, it boils down to the belief system involved in Maslow's approach. Maslow is a humanist and at it's pure expression humanism is not empirical because it sees human beings, not as responders to a behavioristic world, but rather as those who have a freewill and desire to strive for self actualization, an expression of our ultimate creativity, uniqueness and drives. As a result of its nature, Maslow's hierarchy avoids accurate empirical study.
Why is it still so popular despite empirical evidence? The primary reason is that the overall hierarchy, even though simplistic and not based in behaviorism, is in practical terms, seemingly accurate. It is seen in everyday life. For example, according to Maslow, physical and safety needs come before love and belonging needs. How can this be proven? Think of a simple relationship as a good illustration. You go to a party, you meet the person of your dreams. You are having a long and involved wonderful conversation. Suddenly, as you are talking to your dream person, meeting your love and belonging needs, you realize that you have to go to the bathroom. That need, a physical one, will quickly outweigh your love and belonging need and unless you wish to embarrass yourself, you will leave to go to the bathroom.
Despite all of the behavioristic attempts to relegate human behavior to measurable terms, we still exceed measurability on almost a daily basis. Could I really tell you what you are going to do today even though I have observed you for a long period of time? No. But, using Maslow's hierarchy, I have instant knowledge of what will motivate you today.
There are a number of studies that do contradict Maslow's hierarchy, even one as simple as Harlow and Zimmerman's Rhesus Monkey studies on attachment which seem to show that love and belonging needs are dominant over physical needs. But Maslow's hierarchy is simplistic, easily demonstrated in every day life, and can be applied to a large variety of situations and circumstances such as an individual's personal world, business community, and overall social interaction. Basically, it works.
I hope this helps you,