I very recently treated a young lady who is on her third round of chemotherapy with very aggressive drugs. She has been dealing with cancer, tests, medication, basically suffering for the last two years, it was not something I would ever wish to endure myself.
One of my specialities is the treatment of vomiting disorders and this particular young lady was one of the more severe cases I've seen. She was already on a cocktail of antiemetic drugs that had to be administered parenterally because she could literally keep nothing down. She was pale, gaunt and in constant pain; she suffered mood swings and depression; exhaustion, even getting out of bed that morning had been a monumental task.
Her family were falling apart because they could do nothing but stand back and watch her suffer, she felt guilty about shouting at her mother one morning for innocently asking if she wanted a cup of coffee, knowing that she wouldn't be able to keep it down. Her relationship with her long-term partner had reached breaking point, she felt guilty because he had effectively become her carer and was buckling under the strain.
I wouldn't wish that kind of pain on anybody, enduring cancer would simply be switching one despairing existence for another. You may argue "but at least there's an endpoint", perhaps, but with a planned, rational suicide you get to decide that endpoint, with cancer - you're at the mercy of a monster over which you have no little to no control. Sure, you know that you're going to die, but it may be slow, painful, for all involved, and you don't get to make it stop.