Morphine is an opioid, a narcotic. It’s one of the controlled drugs that has the potential of being addictive. Until a decade or so ago, opioids were only really used in the United States for acute pain (such as after surgery) or for cancer pain. After the government relaxed the rules and allowed doctors to prescribe it for chronic pain, some doctors began using it for patients with severe chronic back pain.
Unfortunately, although it seemed that this would be a good solution, there is no proof that opioids actually help pain like chronic back pain. Studies over the past years have failed to find any benefit to taking the opioids and they don’t seem to help workers return to work any quicker than other methods of pain relief. This is likely why your doctor is reluctant to prescribe morphine to you.