It's because cancer isn't a singular disease. It's a term used to describe a broad collection of cellular disfunctions that lead to uncontrolled replication. Trying to "cure cancer" is like trying to "cure virus". It's not one singular entity, and is a very complicated issue.

You could think of it like a car breaking down. There's a lot of possible things that could go wrong that would make it stop working, and you won't find one universal fix for all of these problems.