Apoptosis is the process of Programmed Cell Death (PCD) that occurs in multi cellular organisms.
Apoptosis is a complex and a highly regulated process that kills the cells. There are various biological events that lead to the cell changes and death.
This all happens for a normal cell. But, in a Cancerous cell, the ability to communicate with other cells is lost due to loss of surface proteins.
Also, there is a gene called p53 that checks if the genes in a cell are faulty or not and whether the cell should die. In a cancerous cell, this gene gets faulty and so they neither can't repair themselves nor die. This creates a host of new problems along with others like abnormal growth and competence with other cell etc.
So, that is the most probable cause as to why cancer cells don't undergo Apoptosis.