Why did the Lord choose crucifixion rather than another form of death?
The Lord chose crucifixion because it is a form of death man cannot accomplish on himself. Crucifixion requires outside action—you cannot nail yourself or raise the pole. It pictures that being crucified with Christ is not our work but wholly God's work on our behalf.
Les explains, in the context of Romans 7 and old Adam, that old Adam 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 one form of death man cannot accomplish on his own. You cannot drive the nails into your own hands and put the pole up in the air, because that had to be done by outside. So crucifixion was the death of choice, to carry through the whole theme that as we are crucified with Christ, it is nothing that we can do. It is totally, wholly, and completely a work of God on our behalf.