Why aren't some doctors upfront with cancer patients?

Some people can not handle the truth that is why some doctors say different things to the patient. A real life example; My mother was having this strange cough and started going from one doctor to another. Finally on doctor told my mom that he wanted to see her children in his office. We went into this office, mom stayed outside, and he told us she is dieing of ALS and for us not to let her know about it. Get the picture.