One of the least understood things about cancer is the way that it can go into remission for many years and then flare up again. Just because you were given a clean bill of health after the earlier treatment does not guarantee that you will not fall victim to the same disease again.
When you apply for a life insurance policy, you will have to get a medical exam that test for many different health concerns, including cancer, blood disease, heart disease, and more. If you test positive for any of those things, the insurance company will either charge you much higher rates or deny coverage at all.
For insurance companies, it is all about risks. If you were treated and declared free of the cancer, and your medical exam does not indicate any problems related to cancer, then you will most likely be eligible for life insurance. The thing is, since you have had cancer, you are at a higher risk level than someone who has never had it at all, and your rates are almost certain to be much higher.
In your case, the amount of time that has passed since you were treated for cancer will actually work in your favor. Since you have been cancer free for quite a few years, the risk of the cancer popping back up is much lower than if your treatment was only months previous to the application. Since you are able to show an extended period of being cancer-free, most insurance companies will sell you a policy.
Finally, the insurance company may issue you a policy but exclude the cancer you have previously suffered from the coverage. In this case, you would be covered, but if the cancer returns and proves terminal the insurance company would be exempt from paying on the policy. If you are having trouble finding a life insurance policy because you previously had cancer, picking up a policy of this sort may be your best option for getting good life insurance.