Question: why is yudhisthira called dharmaraj?
Answer: Yudhishtira is considered dharmraj because he was the son of Dharam (also known as Yamraj, lord of death too). And also because he always took path of Dharm. He never lied except in Mahabharata war and that only one white lie. Due to his righteousness he was the only one who was able to reach heaven alive.