It's not clear yet why decompression surgery for cervical myelopathy isn't more successful. Studies done by surgeons with years of experience and good results for other surgeries report very little improvement after this operation.
It may be that patients with other problems are the ones with worse outcomes. Perhaps having heart disease, or diabetes, or a thyroid problem affects the results. Age may be a factor. Some studies show poor results in older adults.
Some surgeons say the type of surgery makes a difference. The amount of cord compression, alignment of the spine, and previous surgery to the area may make a difference. Worse disease is usually operated on from the back (posterior approach). Patients who require more extensive surgery often have worse results.