He (Vidura) did not ask Maharaja Yudhisthira in that way (get out of the false position of King) because he knew that a king like Maharaja Yudhisthira is aware of all the fearful situations of this flimsy world and would take care of himself
SB Canto 1
Mahātmā Vidura knew all this, and therefore he addressed Dhṛtarāṣṭra, saying: My dear King, please get out of here immediately. Do not delay. Just see how fear has overtaken you.
Cruel death cares for no one, be he Dhṛtarāṣṭra or even Mahārāja Yudhiṣṭhira; therefore spiritual instruction, as was given to old Dhṛtarāṣṭra, was equally applicable to younger Mahārāja Yudhiṣṭhira. As a matter of fact, everyone in the royal palace, including the King and his brothers and mother, was raptly attending the lectures. But it was known to Vidura that his instructions were especially meant for Dhṛtarāṣṭra, who was too materialistic. The word rājan is especially addressed to Dhṛtarāṣṭra significantly. Dhṛtarāṣṭra was the eldest son of his father, and therefore according to law he was to be installed on the throne of Hastināpura. But because he was blind from birth, he was disqualified from his rightful claim. But he could not forget the bereavement, and his disappointment was somewhat compensated after the death of Pāṇḍu, his younger brother. His younger brother left behind him some minor children, and Dhṛtarāṣṭra became the natural guardian of them, but at heart he wanted to become the factual king and hand the kingdom over to his own sons, headed by Duryodhana. With all these imperial ambitions, Dhṛtarāṣṭra wanted to become a king, and he contrived all sorts of intrigues in consultation with his brother-in-law Śakuni. But everything failed by the will of the Lord, and at the last stage, even after losing everything, men and money, he wanted to remain as king, being the eldest uncle of Mahārāja Yudhiṣṭhira. Mahārāja Yudhiṣṭhira, as a matter of duty, maintained Dhṛtarāṣṭra in royal honor, and Dhṛtarāṣṭra was happily passing away his numbered days in the illusion of being a king or the royal uncle of King Yudhiṣṭhira. Vidura, as a saint and as the duty-bound affectionate youngest brother of Dhṛtarāṣṭra, wanted to awaken Dhṛtarāṣṭra from his slumber of disease and old age. Vidura therefore sarcastically addressed Dhṛtarāṣṭra as the "King," which he was actually not. Everyone is the servant of eternal time, and therefore no one can be king in this material world. King means the person who can order. The celebrated English king wanted to order time and tide, but the time and tide refused to obey his order. Therefore one is a false king in the material world, and Dhṛtarāṣṭra was particularly reminded of this false position and of the factual fearful happenings which had already approached him at that time. Vidura asked him to get out immediately, if he wanted to be saved from the fearful situation which was approaching him fast. He did not ask Mahārāja Yudhiṣṭhira in that way because he knew that a king like Mahārāja Yudhiṣṭhira is aware of all the fearful situations of this flimsy world and would take care of himself, in due course, even though Vidura might not be present at that time.