Why were the two thieves only tied to their crosses while Jesus was nailed?
There they crucified him, and with him two others, one on either side, with Jesus in the middle (John 19:18).
Two revolutionaries were crucified with him, one on his right and the other on his left (Matt. 27:38).
With him they crucified two revolutionaries, one on his right and one on his left (Mark 15:27).
When they came to the place called the Skull, they crucified him and the criminals there, one on his right, the other on his left (Luke 23:33).
There is nothing in the Gospels that would suggest that the thieves (or revolutionaries) were crucified in a manner different from Jesus'.
The common perception of this comes from popular artwork that depicts the two thieves as being tied in addition to being nailed, whereas Jesus is depicted as nailed to the cross and not tied. This is an artistic expression and not a historical statement. The artistic statement is in regard to the free choice of Jesus to lay down his life (John 10:18). The general idea is that the criminals had to be tied down in order to nail them to the cross, whereas this was not necessary in the case of Jesus because, as the traditional expression goes, sin nailed him to the cross, but love kept him on it.