John Gill Commentary Jeremiah 31:37

"Thus saith Jehovah: If heaven above can be measured, and the foundations of the earth searched out beneath, then will I also cast off all the seed of Israel for all that they have done, saith Jehovah." — Jeremiah 31:37 (ASV)

Thus says the Lord, if heaven above can be measured
Either the space between the highest heavens and the earth; or the extent of the heavens, from one end of them to the other, which cannot be done by man; so the Targum, "as it is impossible that man should know the measure of the heavens above;" otherwise it is measured by the Lord, for he has "meted out heaven with a span", (Isaiah 40:12) ; and the foundations of the earth searched out beneath :
so as to be known what they are, or on what they are fastened, since the earth is hung upon nothing, (Job 38:6) (26:7) ;

I will also cast off all the seed of Israel for all that they have
done, says the Lord ;
as the former cannot be, so neither the latter;

when there was a very great rejection of the Jews for their disbelief of the Messiah, they were not all cast off; the Apostle Paul was an instance to the contrary, and so were others: "the remnant according to the election of grace"; and there is a time coming when all Israel shall be saved, (Romans 11:5Romans 11:25Romans 11:26) ; nor shall any of the spiritual Israel be cast off by him, or cast away from him, so as to perish; the Israel, whom God foreknew, is chosen, redeemed, and whom he calls by his grace; no, not for all the sins and transgressions they have been guilty of, however they may deserve it.

The reasons are, because of his unchangeable love to them; his unalterable covenant with them; the satisfaction his son has made for them; and the free and full pardon of their sins, which he has granted to them.