Why do humans have digestive system present at front and kidneys back?
I don't claim to have the full answer to why these organs are located in the way they are, but I think these factors have an influence on the anatomy: 

1- The urinary system's link with the reproductive system in male mammals: The male reproductive system outlet needs to be located as far to the front as possible in a 4 legged mammal to facilitate sexual function, forcing the urinary system to follow  

2- The kidneys are very fragile and need to be protected by the thick back muscles and partly under the rib cage. They would be too vulnerable at the front

3- The urinary system is generally sterile of bacteria, and so is the reproductive system in males, and partially in females. The digestive system isn't sterile in both. The outlet of the digestive system can't be in the front of the genitals in a male as this will increase the risk of urinary infections overall and also genital infections after mating. It can't be in between the urinary and reproductive outlets for the same reason, so it has to be located behind both to cause the least interference with their functions