Why is cancer so difficult to treat ?

Because every cancer is different. And there are sub types that are different as well. 

Because it grows quickly and if chemotherapy is the weapon of choice, it can destroy good cells as well as the cancer.

Because even if you do go into remission, there is still some cancer there. If ever you want an argument for survival of the fittest, there's your answer. The cells that are left have not been killed off and they reproduce and even mutate. This can happen no matter what the treatment.

Because the patient is worn down by throwing up after every treatment. Very few people survive the constant nausea (maybe this one is a bit iffy. Some people do not feel the effects of the treatment). 

Because normal activities are greatly curtailed. Some cancers drain energy because the red blood cells are not doing what they should. That means oxygen is not getting where it should go to. Tiredness is a real concern in many cancers.