Although it may seem obvious to operate on the back by the back, looks can be deceiving. In the lower back, for example, right near the bottom, it can be hard to access certain discs. While the surgery is possible, the degree of difficult is such that there could be problems with nearby muscles or discs.
Surgeons have learned that they can also approach the back from the abdomen, avoiding much of the problems associated with the lower part of the back. By accessing this way, the surgeons can reach the targeted discs with more ease.
An issue that the surgeons do have to keep in mind is that they want to avoid trauma to any of the internal organs in the abdominal area. They do this, however, with proper pre-operative preparation, so they can see where all the organs are and avoid them while the perform the surgery.