The New Covenant was actually promised to the Jews as an unconditional Covenant and spoken of as such in both the books of Ezekiel and Jeremiah.
31 “Behold, the days are coming, says the Lord, when I will make a new covenant with the house of Israel and with the house of Judah— 32 not according to the covenant that I made with their fathers in the day that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the Lord. 33 But this is the covenant that I will make with the house of Israel after those days, says the Lord: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. 34 No more shall every man teach his neighbor, and every man his brother, saying, ‘Know the Lord,’ for they all shall know Me, from the least of them to the greatest of them, says the Lord. For I will forgive their iniquity, and their sin I will remember no more.” Jeremiah 31:31-34
This wasn't generally taught by reformation theologians because it contradicts their replacement theology. The covenant is not made with Israel alone, but with Judah as well.
16 Therefore say, ‘Thus says the Lord God: “Although I have cast them far off among the Gentiles, and although I have scattered them among the countries, yet I shall be a little sanctuary for them in the countries where they have gone.” ’ 17 Therefore say, ‘Thus says the Lord God: “I will gather you from the peoples, assemble you from the countries where you have been scattered, and I will give you the land of Israel.” ’ 18 And they will go there, and they will take away all its detestable things and all its abominations from there. 19 Then I will give them one heart, and I will put a new spirit within them, and take the stony heart out of their flesh, and give them a heart of flesh, 20 that they may walk in My statutes and keep My judgments and do them; and they shall be My people, and I will be their God. 21 But as for those whose hearts follow the desire for their detestable things and their abominations, I will recompense their deeds on their own heads,” says the Lord God. Ezekiel 11:16-21
The reformers avoided this passage as well in that it makes little sense to say the gentiles were scattered among the gentiles, but refers to the Diaspora of Israel.