Why was the law unable to do what it could not do?
The law could not accomplish its purpose because it was weak through the flesh, hindered from man's fleshly side. Though perfect from God's perspective, it lacked power in humanity's sinful condition, so God sent His own Son in the likeness of sinful flesh to condemn sin in the flesh.
Les explains that the law itself was righteous and perfect from God's point of view, but it was weak and beggarly from man's fleshly side. The problem was not the law but the flesh—old Adam, the sinful nature that is the fountainhead of all our problems. The law could not deliver because it had no power over that Adamic nature. God therefore sent His own Son in the likeness of sinful flesh, to take the place of old Adam, so that we could be crucified with Him and sin condemned in the flesh. Les notes that when Paul speaks of sin singular, he means old Adam; sins plural are the things old Adam causes us to do.
For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh:
That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.