Why was Jesus crucified in the middle?

Background
Prior to Jesus being brought to Pilate, there were three criminals in custody awaiting execution. Since it was the custom to release one, the Romans would have been prepared to crucify the other two. It is speculation, but it is reasonable to believe the two upright pieces of those two crosses had been put in place the day before. Then, after one had been freed, the other two would carry the other piece of their cross to the place where they would be crucified.

Jesus' was a third and unexpected crucifixion, which the Roman soldiers would not have prepared; both pieces of the cross would have to be carried out. This fits the otherwise conflicting accounts of Jesus making the trip to Golgotha. Regardless of the exact details, Jesus' crucifixion was something the Roman soldiers had to deal with "at the last minute."

We know the Roman soldiers mocked Jesus and hailed Him as King, so it is reasonable to conclude they decided to place Him in the middle, which would have been the "position of honor."

Fulfilling Scripture
The Crucifixion and Resurrection are the focal point in God's plan of salvation and nothing was "by chance." On the day of Pentecost Peter says as much:

this Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men. (Acts 2:23) [ESV]

For example, the method of execution could not be by stoning:

Christ redeemed us from the curse of the law by becoming a curse for us—for it is written, “Cursed is everyone who is hanged on a tree” (Galatians 3:13)

“And if a man has committed a crime punishable by death and he is put to death, and you hang him on a tree, his body shall not remain all night on the tree, but you shall bury him the same day, for a hanged man is cursed by God. You shall not defile your land that the LORD your God is giving you for an inheritance. (Deuteronomy 21:22-23)

The people acted in ignorance [of God's plan (Acts 3:17)], but the course of history was guided by God so that His plan would be fulfilled, according to Scripture. Isaiah 53 describes in detail the Suffering Servant. It ends with this passage:

Therefore I will divide him a portion with the many, and he shall divide the spoil with the strong, because he poured out his soul to death and was numbered with the transgressors; yet he bore the sin of many, and makes intercession for the transgressors. (53:12)

He was numbered with the transgressors... "Numbered" is the Hebrew manah which Gesenius' Lexicon says properly means "to be divided, to be divided out, to divide." In this form it can also mean to be reckoned, to be assigned. So by being placed between two transgressors, Jesus "divided" them; actually being "assigned" His position. This was necessary to fulfill Isaiah 53:12.

Also note that Isaiah says He "makes intercession for the transgressors. This was actually fulfilled while on the cross in the singular with the one thief who repented and in the plural:

And Jesus said, “Father, forgive them, for they know not what they do.” (Luke 23:34)

So we might say the plan of salvation was not only fulfilled; it was literally being "acted" out in real-time for the other participants. Similarly, just as Jesus death was a substitution for all sinners, He actually took Barabbas' place.

Finally, the Gospel of John begins with a description of the Word which was with God coming to the world and returning to God. This too follows Isaiah:

⁸ For my thoughts are not your thoughts, neither are your ways my ways, declares the LORD. ⁹ For as the heavens are higher than the earth, so are my ways higher than your ways and my thoughts than your thoughts. ¹⁰ “For as the rain and the snow come down from heaven and do not return there but water the earth, making it bring forth and sprout, giving seed to the sower and bread to the eater, ¹¹ so shall my word be that goes out from my mouth; it shall not return to me empty, but it shall accomplish that which I purpose, and shall succeed in the thing for which I sent it. (Isaiah 55)

The word ...shall not not return to me empty...." The Hebrew רֵיקָם is usually translated as empty-handed:

If the God of my father, the God of Abraham and the Fear of Isaac, had not been on my side, surely now you would have sent me away empty-handed. God saw my affliction and the labor of my hands and rebuked you last night.” (Genesis 31:42)

Again there is a literal fulfillment when one thief had a change of heart:

⁴⁰ But the other rebuked him, saying, “Do you not fear God, since you are under the same sentence of condemnation? ⁴¹ And we indeed justly, for we are receiving the due reward of our deeds; but this man has done nothing wrong.” ⁴² And he said, “Jesus, remember me when you come into your kingdom.” ⁴³ And he said to him, “Truly, I say to you, today you will be with me in paradise.” (Luke 23)

The Word did not return "empty-handed." He brought the penitent thief.

Conclusion
Being crucified between two other transgressors was necessary to fulfill the prophecy of the Suffering Servant. Likewise, the "death bed" request from one of the thieves was necessary, or at least was an immediate fulfillment of the work of the Word as described in Isaiah 55.

While the first two posters have made some good and legitimate points, one observation has yet to be made, and that concerns first, why Jesus was there at Golgotha, and second, what he accomplished by being there.

FULFILLING SCRIPTURE

Clearly, one of the reasons why Jesus allowed himself to be crucified was to fulfill the Scriptures concerning himself (see Luke 24:25-27). Of the many references to Jesus in the Tanakh, Isaiah Chapter 53 and Psalm Chapter 22 have more prophecies concerning the death of Jesus than any other chapters in the entire Bible, and they give us in gruesome detail what Jesus endured on the cross and why he endured it.

Why was Jesus there at the place of a skull? In short, to become a sin offering for sinners. As the hymn writer said,

On a hill far away
Stood an old rugged cross,
The emblem of suffering and shame
And I love that old cross
Where the dearest and best
For a world of lost sinners was slain

At the cross, Jesus was "numbered with the transgressors" (Isaiah 53:12), one at his right side and one at his left. We would not be amiss in thinking that the one malefactor represents unbelieving humanity who choose not to repent, while the other malefactor represents believing humanity who see the necessity of repenting.

DRAWING ALL MEN UNTO HIMSELF

Rhetorically, the two criminals who were crucified with Jesus comprise a synecdoche, with each criminal standing for two categories of people: the saints and the ain'ts! In other words, they have a part-to-whole relationship with the rest of humankind. With Jesus in the middle, each malefactor had equal access to Jesus and could turn to him in the midst of their pain. Today, all people have the same opportunity to repent, since God is not a respecter of persons (Acts 10:34)

Moreover, both criminals heaped insults on Jesus (see Matthew 27:44). One of the rebels, however, had a change of heart and repented.

40 But the . . . [repentant] criminal rebuked him. “Don’t you fear God,” he said, “since you are under the same sentence? 41 We are punished justly, for we are getting what our deeds deserve. But this man has done nothing wrong.” 42 Then he said, “Jesus, remember me when you come into your kingdom.” 43 Jesus answered him, “Truly I tell you, today you will be with me in paradise.”

In conclusion, as gory as the scene at Calvary was, God was still at work in powerful ways. Not only that, God's Son was finishing the work he was sent into the world to finish; namely, to provide redemption to sinners who would simply repent and cast themselves on the mercy of God.

Jesus's second-to-last utterance from the cross was "Finished!" (or "Accomplished!") With that loud cry, the work his Father had sent him to finish was finished. Three days later, he would rise from the dead "with a mighty triumph o'er his foes."

Up from the Grave He Arose (Low in the Grave He Lay)
The United Methodist Hymnal Number 322
Text: Robert Lowry, 1826-1899
Music: Robert Lowry, 1826-1899 Tune: CHRIST AROSE
Meter: 65.64 with Refrain

1. Low in the grave he lay, Jesus my Savior,
   waiting the coming day, Jesus my Lord!

   Refrain:
   Up from the grave he arose;
   with a mighty triumph o'er his foes;
   He arose a victor from the dark domain,
   and he lives forever with his saints to reign.
   He arose! He arose! Hallelujah, Christ arose.

2. Vainly they watch his bed, Jesus my Savior,
   vainly they seal the dead, Jesus my Lord! (Refrain)

3. Death cannot keep its prey, Jesus my Savior;
   He tore the bars away, Jesus my Lord! (Refrain)