Cancer is like a car losing its brakes. Cells normally have brakes in place that prevent them from multiplying. When cells lose these brakes they start multiplying like crazy. When bacteria infect our body they also start multiplying like crazy, but there is one key difference. Bacteria are recognized by our immune system as foreign objects and they are attacked. Cancer cells, on the other hand, are by and large ignored by our immune system because they are identified as self, not as a foreign body.
And because bacteria have a physiology and a biochemistry that is very different from that of our own cells, scientist can use these extreme differences as unique targets to come up with drugs (antibiotics) designed to exclusively affect bacteria. As a result of this, antibiotics don't affect us at all most of the time. Additionally, bacteria also have to deal with the immune system. Thus, the double whammy of antibiotics and immune system normally wipes them out. In fact, many antibiotics don't kill bacteria, they just delay their growth while the immune system catches up and does the killing.
Cancer cells however, are so similar to healthy cells that scientist have very few unique differences that they can exploit to come up with drugs that will only affect the cancerous cells. The difference that is most often employed is the fact that cancer cells multiply very fast while most of our cells don't. But that is the key word, "most". We do have a significant number of cell types in our body that multiply very fast: For example, in our bone marrow, in our intestines, and the cells that make our hair. This is why most anti-cancer drugs also cause damage to the body.
Hence, scientists administering a cancer drug are faced with a balancing act. They must deliver a dose of the drug that is effective against the cancer, but they can't give too much or else they can cause significant toxicity to the patient. Many times the doses necessary to kill the cancer are too high for the patient to withstand, and this is especially true in the case of advanced cancers that have spread. This is why the best way to deal with cancer is to get screened and increase the chance of detecting it early. Mammograms, pap smears, prostate exams, colonoscopies and other such procedures are instrumental to this process.
So you see, in these technologically advanced times the old adage "an ounce of prevention is worth a pound of cure" is still true!