Why do Christians not follow O.T law, when Jesus said he was there to uphold it?

Romans 10:4 says Christ is the end of the law. Obviously, therefore, Jesus was contrasting the endurability of heaven and earth and that of the law "not being fulfilled." Law does not teach righteousness, otherwise, Israel would have been righteous. Rather, law merely restrains wickedness. For example, if law were removed from this country, how would the majority of its citizens react? Would they demonstrate love and goodness or steal and plunder? So when law is removed, the heart is revealed. Because it reveals man's true wickedness. That is why Paul said that governmental law is "God's avenger." It keeps mankind from total chaos. (Romans 13:1-8). So it was with the Mosaic Law. It was also God's means of expressing his attitude toward man's misconduct. However, law does not instill righteousness in the heart. Therefore, the law was temporary. If the Mosaic law were still in effect, Christians would have to perform a number of impractical compliances such as building a fence around the roof of their houses, running to a city of refuge in case of accidental murder, and finding a place outside their residence to "go to the restroom." (Deuteronomy 22:8,Numbers 35:6, Deuteronomy 23:12, 13) We no longer live by the letter of the law but by the spirit of the law, the principles behind the law. When a street light goes out, do we run the light? Hardly! We recognize the principle of respect for life and so slow down and check for traffic. Likewise, we may not have the law but operate by the principles behind them. Righteousness is a streetlight with green commandments that instrct us what to do and red laws that tell us what not to do. But it is the yellow principles that reveal the heart. Whereas it is intended as a caution to slow down many use it as an excuse to speed up. And so rather than yield to godly principles, many excuse themselves and run to do badness. Regardless, God is allowing what les hearts to be revealed so that when he arrives to judge, man will have shown his true nature.