Here we go with the dumb conspiracy theories.
Cancer is hard to "cure" because it's not a single disease, it's an umbrella term for hundreds of different conditions characterized by abnormal cell growth. The level of complexity of cancer is huge...you have a bunch of different types of a certain cancer (ie. lymphomas), and even then each person's specific one is different.
What makes cancer really hard to treat is that the body can't distinguish between normal cells and cancer cells, so you have to try to find a way to
just kill cancer cells without killing healthy cells - it's hard. Most things that would kill cancer cells effectively will also kill healthy cells. People have compared it to trying to win a war without killing any of your own men, but both sides are dressed the same and the only difference is that some of the enemy soldiers have slightly longer shoestrings or slightly different colored hats, and everybody can change clothes mid-battle.
Plus, as cancer cells divide, the new cancer cells keep mutating, so treatments aimed at killing specific cells might leave some alive due to mutating. And if you leave even a couple cancer cells alive, the tumor can just regrow.