I think The Lord explains His reasons pretty thoroughly in Matthew 23.
I believe the one that most sums it up is in verses 25 and 26: “Woe to you, scribes and Pharisees, hypocrites! For you cleanse the outside of the cup and dish, but inside they are full of extortion and self-indulgence. Blind Pharisee, first cleanse the inside of the cup and dish, that the outside of them may be clean also."
So not only did they follow the law by letter alone and not the by its spirit, but this implies that they didn't even
follow it by the letter. Rather, they were more preoccupied with making themselves appear
to be righteous. This was Christ's sticking point in his criticism of the Pharisees.