I honestly don´t know. But here are some possible reasons your doctor may not want you to consult an osteopath for low back pain / sciatica.
Your doctor doesn´t really know what osteopathy is, or what osteopaths do
If you didn´t know anything about a profession you might distrust it, and you almost certainly wouldn´t recommend that your clients consult a member of that profession. Doctors care about their clients/patients and for the most part feel responsible for their wellbeing and don´t like to take chances.
Your doctor has a bias
We all have biases, and unfortunately these tend to be predominantly negative. If your doctor has ever heard anything bad about an osteopath, he/she is more likely to remember that than anything positive.
Your doctor has valid concerns
Your doctor may feel that you would be better-off pursuing another course of action for your problem.
If you´re concerned whether osteopathy is right for your lower back pain or sciatica, here are a few articles you should find useful: