I’m sorry, but this seems like a bit of a silly question. A group includes in its mythology that it is the chosen of a god and you want an answer to why a god needs to choose anyone? What sort of answer would you expect?
To the extent that there is a “why?” answer, it would probably come down to the story of Abraham, in which the Lord finds a completely faithful man and rewards him for that faith.
Later generations consider that promise from the Lord and come to the conclusion that the choice places an obligation on them to commit to righteousness.
The only real difference between their mythology and the mythologies of other nations such as Great Britain or the United States is that they actually expended some effort to come up with the theme of obligation, unlike themes in which other societies seem to indulge (such as Manifest Destiny).