Cancer is a phenomena in which cells begin to reproduce at an increased rate and with higher rates of mutation.
In multi-cellular organisms this behavior is extremely disadvantageous often leading to death. I think that given the wide ranging and subtle list of problems that evolution has solved in the past, the reason that evolution has not built a defense against Cancer may not be because it has been unable to, but rather because it has not needed to.
It seems that a single cellular organism which had a mechanism by which it could detect whether it is being subject to an evolutionarily unfamiliar substrate and then respond to that situation by increasing its rate of reproduction and rate of mutation accordingly would have a greater chance of survival.
Evolution is not simply a mechanism for coming up with an answer for a question, it is a mechanism for answering a question that is constantly changing. To some degree life spans, reproduction rates and mutation rates are a function of the rate of change of the environment. For a given environmental rate of change, the optimal life spans, reproduction rates and mutation rates may be fixed entities.
However in a situation where the rate of change of the environment itself is changing, it would be highly advantageous for an organism to be able to adjust these variables on the fly and across generations. Basically there is impetuous for the storage of a variable that spans an individual and even spans time.
Cancer seems to me to be just such a mechanism. It seems to be a cross generational trait which allows a single cell organism to tune its reproduction and mutation rate in response to environmental factors which indicate that it is experiencing a changing environment.
Perhaps the degree of change required to set an organism into a cancerous state is large enough and the environmental surroundings of a single cell within a multi-cellular animal constant enough, so that usually this theoretically ancient trait rarely exhibits itself within a multi-cellular animal.
However, given the recent events of human civilization, perhaps for the first time, single cells within multi-cellular organisms are interacting with substrates unfamiliar to it to the degree necessary to flip it into a Cancerous state; this due to the increasing use of chemicals not familiar to the multi-cellular animals on a evolutionary scale.