Why was crucifixion the death of choice in connection with being crucified with Christ?
Crucifixion was the death of choice because it is a form of death man cannot accomplish on his own. A person cannot drive the nails into his own hands or raise the pole; it must be done by outside action. This illustrates that being crucified with Christ is nothing we can do, but wholly God's work.
Les explains that Romans 7:5 continues Paul's theme of breaking the relationship of old Adam, who has to be crucified, not by anything we do. We cannot crucify ourselves. Les suggests this is one reason the Lord chose crucifixion rather than any other form of death: it is a death man cannot accomplish on his own. You cannot drive the nails into your hands and put the pole up in the air, because that had to be done by outside action. So crucifixion was the death of choice to carry through the whole theme that as we are crucified with Christ, it is nothing we can do. We cannot crucify ourselves; it is totally, wholly, and completely a work of God on our behalf. Paul then reviews that when we were in the flesh, under control of old Adam, the motions or results of sins, plural, were at work. Sin is old Adam, the fountainhead and manufacturing point; what we do are sins, plural.