"Why were the two robbers who were crucified with Jesus held up  with ropes, while Jesus was nailed to the cross? Does being roped to the cross make a person die?"

Your description of the crucifixion scene comes from old paintings, movies, and Sunday School illustrations.  But that is not the way the Bible describes the crucifixion.  The old paintings are wrong on two points. 

(1) The two criminals crucified with Jesus got the same treatment that He did.  It is not likely that any of them were held up by ropes.  That's not how the Romans did it, and the Bible does not even suggest that they did it in this case.

(2) In 1968 archeologists discovered the bones of a Jewish person named Yehohanan Ben Hagkol -- John son of Hagkol, who had been crucified.   The victim had been nailed to a cross made of olive wood.  One of the seven-inch spikes hit a hard knot in the wood and bent, forming a hook.  The executioners could not remove the nail from the victim's dead body, so he was buried with the nail still in him. 

Archeologists found a wooden plate like a large square washer below the head of the nail, to insure that the victim could not work his way off the nail.  When they closely examined the bones, they discovered that Yehohanan had been crucified with three large nails: one for each arm, piercing the end of the forearms, against the wrist bones (not in the palm of the hands as we normally picture it), and one which pierced both heel bones together (not in the top of the foot as we normally picture it).  

Yehohanan's leg bones had been broken, probably by a club hitting both legs at the same time while he hung on the cross.  This prevented the victim from "standing" on the heel nail in order to breathe and to get relief for his arms.  Breaking the legs hastened death.  Soldiers who crucified Jesus had planned to break His legs, but when they saw that He was already dead, they cancelled that plan. (See John 19.31-33)

[reference:  In the Fullness of Time by Dr. Paul L. Meier, page 165]