The Jews, the nation of Israel, have always been important to God because God made an agreement — a covenant — with one of their ancient ancestors, Abraham (Gen. 15:4-6). Because God loves Israel and is bound to her, he has, throughout history, sent prophets and brought saviours to help his people, Israel (e.g. 2 Chron. 24:19; Jer. 7:24; 25:4; 26:5; 29:19; 35:14-15; 44:4; Zech. 7:12; Judg. 2:16). Jesus, God’s son, is the greatest of these saviours and prophets (Deut. 18:15,18; Acts 3:20,22; Heb. 1:1-2). So, in a long line of saviours and prophets — in fact, as the climax of this line — Jesus’ primary mission was to Israel. God sent him to save Israel.
Of course, it was always God’s plan that, working with Israel, he would also save the members of other nations (i.e. non-Jews, Gentiles): see, e.g. Isa. 49:6; Acts 3:25; Rom. 15:8-12. And, when Israel rejected Jesus, the mission turned to the Gentiles more directly (Acts 13:45-48; Rom. 11:11ff.).