Because - reminding all Christian preachers - the Pharisees are the origins of rabbinic Judaism.
The Pharisee would have been a hero, and the tax collector the worst of traitors and collaborators.
People hearing this story would have been cheering for him, wanting to emulate him. He's a good guy.
Jesus' friends would be steamed when that guy shows up.
TEXT TRANSLATION ALERT!
They are BOTH justified. Alongside one another. Pharisee and tax collector alike, side-by-side, gifted with mercy.
And paradoxical parables are always the work of the Paraclete. 😉❤️