Based on studies performed, the use of mechanical traction as you described just hasn’t been proven effective for the majority of back pain sufferers. Some therapists still do use traction but they are more selective in who they use it on.
For example, patients with numbness or pain down the leg may be helped by traction. There are a couple of special tests that can be done to see who might benefit from mechanical traction. One is the prone extension test. If the leg symptoms go away or move up to the low back area while lying on your stomach and pushing the upper part of the body up (propped on your forearms), then traction might be indicated.
Another predictive test is a crossed straight leg raise. If the therapist raises the uninvolved leg straight up (while you are lying on your back) and the pain, numbness, or other symptoms increase on the involved side, then again, you might be a good candidate for traction.
There’s still much we don’t know about low back pain but research is helping us inch forward in identifying the type of treatment that will yield the best results quickly. Traction has its place in that scheme but it’s no longer the main treatment offered to everyone with back pain.