Devapi left the kingdom of his father and went to the forest, and therefore Santanu became the king

Expressions researched:
"Devapi left the kingdom of his father and went to the forest, and therefore Santanu became the king" |"When Devapi retired to the forest, his younger brother Santanu became the king"

SB Canto 9

When Devāpi retired to the forest, his younger brother Śāntanu became the king.

SB 9.22 Summary:

The sons of Pratīpa were Devāpi, Śāntanu and Bāhlīka. When Devāpi retired to the forest, his younger brother Śāntanu became the king. Although Śāntanu, being younger, was not eligible to occupy the throne, he disregarded his elder brother. Consequently, there was no rainfall for twelve years. Following the advice of the brāhmaṇas, Śāntanu was ready to return the kingdom to Devāpi, but by the intrigue of Śāntanu's minister, Devāpi became unfit to be king. Therefore Śāntanu resumed charge of the kingdom, and rain fell properly during his regime. By mystic power, Devāpi still lives in the village known as Kalāpa-grāma. In this Kali-yuga, when the descendants of Soma known as the candra-vaṁśa (the lunar dynasty) die out, Devāpi, at the beginning of Satya-yuga, will reestablish the dynasty of the moon.

Devāpi left the kingdom of his father and went to the forest, and therefore Śāntanu became the king.

SB 9.22.12-13, Translation:

The sons of Pratīpa were Devāpi, Śāntanu and Bāhlīka. Devāpi left the kingdom of his father and went to the forest, and therefore Śāntanu became the king. Śāntanu, who in his previous birth was known as Mahābhiṣa, had the ability to transform anyone from old age to youth simply by touching that person with his hands.