.Why were the Israelites unable to gain right standing in the sight of God?...

Because they did not pursue righteousness by exercising faith in God and His purposes but by attempting to follow all the details of the Law of Moses apart from the Presence of God. They were blindly adhering to the multitude of traditions that had been developed by the teachers and elders of Israel.

Jesus informed the Jews that they were straining out gnats and swallowing camels. They were careful to observe details of tradition but were ignoring justice, mercy, and faith.

There was nothing wrong with the Law of Moses. The problem was, the Israelites did not join heartfelt faith and the Presence of the Lord to the ceremonial observances.