Why wouldn't I?
Galatians 4
Paul tells us that Mt Sinai and the old Jerusalem on planet earth represented the old covenant.
(Gal 4:25) Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children.
In the next verse, Paul tells us the New Jerusalem represents the New Covenant, and is our mother (Sarah represents freedom)
(Gal 4:26) But the Jerusalem that is above is free, and she is our mother.
Paul then tells his audience that they are like Isaac, children of the free woman, not the slave woman
(Gal 4:31) Therefore, brothers and sisters, we are not children of the slave woman, but of the free woman.
Notice how Paul tells us that the BOC is like Isaac.
I don't know how you can read Gal 4 and still deny that the BOC is not under the New Covenant.