In order to fulfill the promises made to them [to Israel].
And here (among other passages):
25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in [G1525 - eiselthe / eiserchomai].
[the "fulness of the Gentiles" being a distinct phrase and meaning from "the TIMES of the Gentiles" ('Gentile domination over Israel' reflected in Neb's 'statue/image' dream, with Neb as 'head of gold' and which started in 606bc and ends at end of trib, per Rev11:1)]
26 And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:
27 For this is my covenant unto them, when I shall take away their sins.
28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes.
29 For the gifts and calling of God are without repentance [/irrevocable].
[the section of Romans 9-11 being about "nations": "Israel [singular nation]" and "the Gentiles [plural nations]"]