The Pharisees had such strict standards because they were reactionaries and visionaries. They were reactionaries in the sense that as they looked at the history of the Jewish people they saw a pattern of disobedience = punishment/faithfulness = blessing. They reacted to the history of their people by establishing stricter standards of the Law on people which basically started with good intentions but ended up something entirely different as it went through these shifts (I cannot really document this but this is how I think they got to the point we see them in the New Testament and passages like Mark 7:1-23):
LAW only —- LAW + tradition —- LAW/TRADITION —- TRADITION + law
They were visionaries in the sense that they believed if they could get all God’s people to adhere to a stricter code of moral and ethical conduct then God would deliver his people. Roman rule was seen as a result of their disobedience. The remedy was strict obedience to the highest standard. In essence, they believed if all the people adhered to the rules of the priesthood then God would be pleased with their obedience and fix their problem with the Romans and re-establish them in the land. This basically makes the Pharisees the first proponents of the “priesthood of all believers” in that they applied the laws only designed for the priests to everyone and on top of that added the traditions of the elders.
It is one thing to apply a stricter standard to yourself. It is quite another to apply that standard to others and basically say they must not be God’s people unless they jump through the hoops of your own tradition. Generation after generation has struggled with keeping a higher standard (monastics, restorationists, emergents) without saying you aren’t really a Christian unless you keep our rules.