The operation you had is called an anterior cervical discectomy and fusion (ACDF). It's widely used by many surgeons to treat cervical (neck) problems. Motion at the joint after ACDF is called pseudoarthrosis. Unfortunately it is all too common a result.
Research shows that pseudoarthrosis occurs in up to 20 percent of single-level fusions. The failure rate goes up to 50 percent with multiple-level fusions. There's no clear reason why this happens. Studies do show that fusion at any level results in breakdown above and below the level of the fusion. With more than one level fused, there may be much more stress and load on the spine in and around the site of the fusion.
The surgeon's level of experience may be a small factor. The number of complications does decrease the more operations a surgeon does. This is all part of the normal learning curve. No surgeon performs this operation without considerable training and practice first.
Others suggest diet, nutrition, tobacco use, and overall health may make a difference. It could be a combination of things that differ from patient to patient. More studies are needed to clear up this confusion.