Why? Because he was busy trying new age garbage to try and treat it. Guess how that worked out for him.
It turns out that doctors found the the tumor in October of 2003 during a routine scan. While a biopsy revealed that the cancer was a rare but treatable form, Jobs opted to try and treat the cancer with a special diet. Jobs is a long-time Buddhist and vegetarian, and though research has shown diet to be effective in reversing coronary disease and even slow prostate cancer, surgery is so effective for this type of cancer that most patients live 10 years or more after treatment. Jobs tried treating the cancer with the special diet for nine months until follow-up scans showed the tumor was growing. He then had the surgery in July of 2004, when most of us found out about it, and after a relatively short recovery was back at Apple.
This is exactly why I hate woo-woo stuff with a passion. It can not only offer false hope to people but it can actually hurt or kill them. Rational thinking is underrated.