I think it is because our God is a personal God. He is immanent and transcendent. He knows and can be known. He chooses to reach out to us and relate to us through covenant relationship. Because God chooses to relate to us through covenant, we can have full assurance and faith that He will keep His promises. We know that He never changes. Even God’s names reveal His covenant promises:
Jehovah-Jireh: “the Lord will provide” (Genesis 22:14), "My God shall supply all your need according . . . " (Phil. 4:19)
Jehovah-Nissi: “the Lord our Banner” (Ex. 17:15), “His banner over me was love” (Sg. 2:4), “Greater love hath no man than this . . .” (Jn. 15:13)
Jehovah-Shalom: “the Lord our Peace” (Jgs. 6:24), “For he is our peace” (Eph. 2:14)
Jehovah-Raah: “the Lord our Shepherd” (Ps. 23:1), “I am the good shepherd” (Jn. 10:11)
Jehovah-Tsidkenu: “the Lord our Righteousness” (Jer. 23:6), “Christ Jesus, who of God is made unto us . . . righteousness” (1 Cor. 1:30)
Jehovah-Shammah: “the Lord is present” (Ez. 48:35), “I will never leave thee, nor forsake thee” (Heb. 13:5)
Jehovah-Rapha: “the Lord thy Physician” (Ez. 15:26), “The prayer of faith shall save the sick, and the Lord shall raise him up” (Jas. 5:15)
God never changes, and He keeps His promises–even when we don’t.