As the other answers suggest, there are lots of reasons why God has a chosen people.
Just want to add few points.
Lets go back to the creation where God decided or chose to create heaven an earth. He also chose to create humanity to populate the earth. And then begins a privileged relationship between God and his creation. In this relation was a covenant between God and Man. But this relationship is broken by sin and so is the covenant. As time goes, the population on earth increases along with deviant behaviors.
I believe God didn't want His first intention in creation to be lost. So He chose a man with whom He would have a covenant as Genesis 12 says:
The Lord had said to Abram,
“Leave your country, your people and your father’s household and go to the land I will show you”
“I will make you into a great nation and I will bless you;
I will make your name great, and you will be a blessing.
I will bless those who bless you, and whoever curses you I will curse;
and all peoples on earth will be blessed through you.”
And through that chosen person, God would reach out to the entire humanity.
Jesus is the finalization of God's purpose, which is to have a new covenant with not only His chosen people (Israel) but to all humanity, which is resumed by John 3:16 - 17
For God so loved the world that He gave His only begotten son, so that whoever
believes in Him, should not perish but have everlasting life.
For God did not send his Son into the world to condemn the world, but to save the world through him
So through Abraham, God intended to restore His covenant with His creation.