Short, incomplete answer: Cancer evolves. After a therapy has failed, you've effectively selected the proportion of the cancer most resistant to that therapy.
cancer in general an umbrella term for 100s of different diseases.
If the B-specific receptors are gone on your B-cell lymphoma, there may not be a great target left that isn't also on a bunch of cells you need.
Each thing you do that targets "weird" characteristics of cancer (or bacterial infection, or pests) selects for cancer (or bacteria, or pests) that is presenting as less weird relative to everything else around.