Why did the law become weak and beggarly according to Les?
The law became weak and beggarly because fallen man in the flesh, apart from the Holy Spirit, could not keep God's perfect law. Though holy and perfect, the law had no inherent power to enable or help the law keeper obey it, so Israel constantly chafed under it as a yoke.
Les explains that the law itself was holy and perfect from God's point of view, but it became weak, beggarly, and a yoke because it had no power to impart to man to keep it. Man without the power of the Holy Spirit, in his own flesh, could not begin to keep that perfect law, and the law had no power within it to help the law keeper keep the law. Consequently, Israel could not keep it, not even come close. Les contrasts this with grace, saying that under grace believers have something far better: the righteousness of the law can be fulfilled in them as Grace Age believers because of the power of the Holy Spirit. He also notes that in Hebrews, Paul lays out that what was good under the law is far better under grace.