Several reasons:

1. Conventional chemotherapy may still be more effective in treating the cancer unless it becomes treatment resistant.

2. Along similar lines, since conventional therapy has been out longer, there is generally more evidence for treatment efficacy for certain kinds of cancers, especially as first line.

3. Immunotherapy is very expensive. Insurance may not cover certain therapy as first line therapy unless you have failed other treatments. An older drug like paclitaxel or carboplatin may cost $100 per dose, while a common immunotherapy drug like pembrolizumab may cost $30,000 (in the US).

4. While generally immunotherapy is better tolerated with fewer side effects, it is still has significant side effects.

The other important concept to note is that MOST cancers aren’t an urgent medical condition. You have time to try different therapies and holding off on treatment with immunotherapy until the cancer “progresses” generally doesn’t make it less effective (it’s more complicated but for simplicity).

Cancer “progressing” could mean that it is becoming resistant to the current drugs being used, in which switching to immunotherapy at that point wouldn’t make it less effective than starting with it.

Cancer “progressing” to an advanced stage also doesn’t necessarily mean it would be less effective because cancer treatment is based on guidelines which is expert consensus based on studies. For a particular cancer, if immunotherapy is started when the cancer progresses to stage IV, it could mean that it’s because that drug or immunotherapy was only studied in patients with stage IV cancer, so the guidelines only recommend initiation at that point.

For more aggressive cancers, they may start with a more aggressive treatment approach from the beginning.