Before Jesus was led away to be crucified, he was flogged by the Roman soldiers (Mark 15:15). That means he was whipped with leather cords that had pieces of bone or metal tied in them that would rip the flesh off one’s back.1
As a result of this, Jesus' back would be lacerated and he would be in a lot of pain. Depending on the severity of the flogging he received, this would explain why he was not strong enough to carry his own cross and why the Roman soldiers forced Simon to do so. Although Jesus started out carrying his cross (John 19:17), the pain caused by the heavy beam and his weakness following the beatings he received meant he did not have the physical strength to continue carrying the cross.2 Normally, condemned men carried the crossbeam to the site of the crucifixion where it was then fastened to a vertical beam already firmly fixed in the ground.3
21 And they compelled a passerby, Simon of Cyrene, who was coming in from the country, the father of Alexander and Rufus, to carry his cross.