Because he was righteous to his mother. Umar Ibn Al Khattab, being who he is, is told to go to this man because he was righteous to his mother. Now some books of history mentioned a story - Allah knows how authentic it is but it is mentioned in our classical sources that Owais Al - Qarni was the only son, his father had passed away... so he's taking care of his mother in a very dutiful manner and he's asks and begs permission to go to Madinah to meet the Prophet (pbuh).
His mother grudgingly allows him to go but she says, 'look, you know that I need you, therefore I only give you, basically a day or two in Madinah; as soon as you get there, you meet the Prophet (pbu), you come back.' It is said that Owais Al - Qarni traveled from yemen all the way to Madinah and when he got there, lo and behold the Prophet (SAW) was away on an expedition, He wasn't in Madinah so now he is stuck between two options:
The first is to wait a week or two - he's traveled from Yemen, that's already a month journey. The first is to wait a week or two - the Prophet Muhammad (saw) wouldn't be gone for more than a week or two and wait for the Prophet to return. And if he did, he would be raised in status from a tabi'i to a Sahaabi (Companion of the Prophet (saw)). If he did, his status would forever be raised eternally. And the second option was, 'listen to my mother. What did he choose to do?
We can guess. He chose to obey his mother. To give up this voluntary status because being a Sahaabi is a privilege but it's not wajib (not compulsory); listening to your mother is wajib (Compulsory) and so according to this riwaaya, Owais Al - Qarni stayed a day or two, as much as his mother had allowed him and then he rode his mount and he rolled all the way back to Yemen and he voluntarily, willingly, gave up that status in order to please his mother.
It is no surprise, therefore, that the Prophet (sallallahu alayhi wasalam) said what he said to Umar Ibn Al - Khattab (ra) that when Owais Al - Qarni comes to you, ask him to make dua to Allah (subhanho wa taala) because this is a man who used to treat his mother with righteousness, with birr and this really shows us, we want our Dua to be accepted, let us treat our parents with that dignity and respect that they deserve.
We went Allah to bless us? We want to the Prophet Muhammad (saw) to know who we are? Owais Al - Qarni and the Prophet Muhammad (saw) they never met! Allah's (Revelation) came down; Jibreel told him who Owais Al - Qarni was. How magnificent of a status was this man that Jibreel comes down to inform our Prophet (saw) of this human being, whose only good that we know of - we don't know anything else about him, that's all we know - he was barran, he was somebody who would treat his mother with that type of Birr and that is why the Prophet (sallallahu alayhi wa salam) never ceased to remind us and advised us to take care of our parents with the utmost gentleness.
(May Allah allow us to be kind, compassionate and caring towards our parents) Ameen.