Why was the old Adam crucified?
The old Adam was crucified because God did it on our behalf, placing it on the cross in His mind. Christ fulfilled God's demand that the soul that sins must surely die by dying our death, so we identify with His accomplished work; this enables regeneration, restored fellowship, and the indwelling Holy Spirit.
Les explains that when the Holy Spirit enlightens our understanding and regenerates the old dead spirit, we realize that we have died. The ego, the I, the old Adam was crucified and placed on the cross in the mind of God because God did it on our behalf. As soon as we do that, He regenerates our spirit by that outside power, fellowship is restored, and He comes in to dwell in the person of the Holy Spirit. The old sin nature then becomes a new divine nature, now in consort with the eternal God, and it begins to affect the body and the Christian walk, growing in grace and knowledge. Les points back to Romans 6, where Paul makes plain that we must identify with what Christ accomplished on our behalf on the cross; Christ fulfilled that demand of God that the soul that sinneth must surely die, and He died my death.