You’re the first person to use the proper scientific word apoptosis when asking me a question, good work!
I’m not sure whether you mean “why do cancer cells need to evade apoptosis?” or “how do cancer cells evade apoptosis?”, so I’ll try and answer both!
Why do cancer cells need to evade apoptosis? Cancer cells are cells with mutations that make them grow faster than normal and in an uncontrolled way. The body has safeguards to try and stop things like that happening, so normally the kind of mutations that would cause the cells to grow out of control would trigger apoptosis and the cell would die instead of going on to cause cancer. So, only cells that also get mutation that help them avoid apoptosis can survive and go on to cause cancer.
How do cancer cells evade apoptosis? Well, mostly we don’t know, and I’m trying to find out. But in a few cases we do know how the cancer cells manage not to die, and here’s an example:
When apoptosis is triggered, molecules called ‘caspases’ become activated in the cell, and they chew up lots of other molecules to dismantle the cell in a controlled, orderly way. There are some other molecules that help the caspases become active, and in some cancer cells those molecules stop working properly, so the caspases can’t become activated and do their job, so the cell doesn’t die.