Cancer treatment is VERY dangerous. In many cases it might kill faster than illness itself. Meanwhile early stages of cancer are often quite responsive to lighter drugs.

As a rule of thumbs - you want to treat, not to kill. And thus you will not use potentially lethal treatment before you tried safer options and cancer development pattern gives you time for such trials.

You first do what you know works. If it doesn't then you progress to treatments which are more speculative or have more harmful side effects.

You don't go rabbit hunting with an AC130. Use the correct tool for the job or you could easily go overkill

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.

It’s basically risk management. Lots of things in life, including procedures, add risk. If they don’t need those options yet, the less risky path is chosen to (hopefully) find the same reward

This is a bit like asking, "Why do firefighters wait until there's an alarm to send the fire truck?" If cancer is caught early and the patient has a good chance of recovery and survival with the least aggressive treatment, then there's no reason to bring out the big guns which usually come with bigger risks.

Doctors don't typically let cancer progress without treatment. They treat the cancer, based on how far it has progressed and whether it's responded to treatment.

Early on, a cancer is contained in one spot, and it's almost always simpler to cut it out and be done with it. Maybe, follow up with a little chemotherapy (possibly localized to the spot) to be sure.

If swinging a scalpel seems like a bad idea (maybe in a sensitive part of the brain or parts of the neck), then radiation might be a good choice. Radiation is often also used to destroy bone marrow for blood cancers that are addressed with a marrow transplant.

Once cancer spreads, then surgery starts to be less of an option (you can only cut someone up so much, and you'd simply be playing "whac-a-mole" trying to get the tumors), you typically look at chemotherapies that go all over the body. They can be very tough as a treatment, but also effective.

Those are all well established and understood therapies that any hospital that treats cancer patients will be able to provide, and the doctors know how to use all those things to get the best possible outcomes.

Immunotherapies tend to either be very specific, so only usuable in certain situations, or they are personalized and need a facility that has the technology and people to implement them. Many are also novel and still being tested, so they are only available as part of a clinical trial, and a person needs to qualify and the trial needs to still be accepting new patients. Being new, it's also VERY expensive and if many insurance plans only cover it if other treatments haven't worked.

Side Effects.

Most cancer treatment falls in the basket of “Kill the cancer before it kills the patient”.

Immunotherapy is a new type of drug technology. The drugs are new with a ton of nasty and unexpected side effects.

Doctors technically can prescribe any drug for any reason. However, they usually prefer to only prescribe drugs where the drug has had a clinical trial to show that the drug works in that situation. Similarly, the FDA won't approve a drug for use in a particular situation, unless there is a clinical trial showing that the drug is safe and effective in that situation.

The problem is getting clinical trials done. If a drug is brand new, with a ton of side effects and quite likely many unknown serious side effects, who is going to volunteer for a trial. Not just that, but is it ethical for a doctor to do a trial on a pore tally dangerous treatment, if there is already something which works.

When it comes to new drugs, the first clinical trials to be done tend to be done in situations where there is nothing else available. In this case, it is ethical to get volunteers for the new treatment, as the only alternative is no treatment.

Most of the clinical trials for immunotherapy fall into this category, although as doctors get more experience, and the drugs are better understood, there are some clinical trials looking at earlier use being prepared or are underway.