Because we said so!
Behr is a low grade paint that is offered as a cheap product. Why?.. that is what Home Depot's market is... Paint is a combination of its components. Better components cost much more. Paint consists of three main components" A pigment, a binder the glues the pigment to a surface as the paint dries and a solvent that makes the moisture loose enough to brush on. Cheaper paints use synthetic colorants and good paints use organic materials which hide better and cost much more. Behr use PVA resins ( polyvinyl acetate) whereas a quality paint uses the twice the cost PMMA resins (polymethyl methacrylate) Resins are the binder in the paint so Behr simply will not stick to the wall as well.
I could go on an on, but you should start to see where I am going with this. Now for the bathroom paint. I would use something like Zinsser's PermaWhite. You see, the way this paint works is that it has a powdered mildewcide dissolved in it. That mildewcide has a very high solubility in water, and it's that affinity for water that actually makes it MOVE toward the surface of the paint when there's water on the surface of the paint, or even when there's very high humidity in the air. As it gets to the surface of the paint, it kills any mildew spores on the paint surface before they have a chance to grow, thereby keeping the paint mildew free. Again a cheap paint contains very litlle of this compound, thus will offer protection a much shorter time.
Behr will yield a okay result 50% of the time at best. Any adverse conditon will cause it to fail. It will not be as easy to apply, will not last as long and is this is why no professionals will use it.