Why is treatment with monoclonal antibodies better than using chemo or radiotherapy?

Not always: Monoclonal antibodies are just one class of drugs used to fight certain cancers. Often, they have to be used with chemotherapy to get the best results (examples: Rituxan with chemotherapy in some lymphomas or Avastin (bevacizumab) with chemotherapy in colon cancer). Sometimes monoclonal antibodies can be used by themselves also. They aren't necessarily better than chemotherapy, just different.