Why Don't All Surgeons Offer Dual Plane Placement of the Breast Implants?

Honestly, I think most doctors who do breast augmentation do the dual plane procedure, and always have, but have never called it dual plane.
In other words, dual plane augmentation mammaplasty is primarily a marketing term. The standard modern teaching and text books all recommend that the lower insertions of the pectoralis muscle be detached to allow the bottom of the implant to be in the subcutaneous or subglandular position, and the upper pole to be beneath the muscle.
This does allow an upward rotation or lift of the breast, which is very helpful in breasts that have a little sag.
I have for many years used a technique which can more accurately be described as dual plane and which does help lift the breast.
I elevate the entire breast off of the underlying pectoralis muscle-essentially create a subglandular pocket. Then I do a subpectoral breast augmentation.
The breast which has been detached form the muscle then is able to move upward and redrape nicely over the submuscular implant. This operation, elevating 2 planes, the subglandular and subpectoral plane, really helps avoid the need for a breast lift in patients who have had sagging breasts following pregnancy.
See before and after pictures of my "dual plane" breast augmentation.

An under muscle implant is always a dual plane implant. The pectoralis muscle is a big triangle. At least 1/2 of the implant sticks out from the edge of that muscle -- it's under muscle for part of it, but about half has no muscle coverage. 

Problem is most patients and surgeons don't really know what the term dual plane means. As I mentioned in another post , a pocket is created beneath the pectoralis muscle , the muscle covers the top part of the implant and the bottom half of the implant is covered by your breast. Then there is an additional plane of dissection above the muscle for several centimeters. Thus " dual plane ". 

This procedure has been described to be used in patients that have somewhat droopy breasts. The idea is that by dissecting in a plane above the muscle the implant will not be as restricted by the muscle and  be able to fill out the overlying loose breast tissue.  There are other ways besides dual plane to free the muscle in this subset of patients.

But as mentioned in several of the answers below to go in and ask if your surgeon does dual plane is silly. Just ask either above or below the muscle. Most real plastic surgeons will place the implants under the muscle, meaning the pectoralis muscle.

Because honestly is someone telling you they use dual plane going to be the deciding factor in choosing a surgeon. I hope not. The number one deciding factor should be what kind of work does the doctor do. Best thing you can do is go to a board certified plastic surgeon and see what there work looks like with photos.