Answer:
Testicular cancer occurs if a cell divides incorrectly and then passes this mutation on to its daughter cells. Testicles are prone to cancer because they are constantly dividing and constantly making daughter cells, so are prone to making mistakes occasionally.
Some of the main risk factors for developing testicular cancer, other than having testicles, is a condition called cryptorchidism. This basically means your testicles didn't descend on their own. The longer they stay in the abdomen, the more chance they can develop into cancer. Other risk factors are having had mumps, having had an inguinal hernia, and a sedentary lifestyle.
But the number one cause of testicular cancer is just plain bad luck, with no specific risk factor.