Go on then, I'm listening.
If you study the Old Covenant, you will find that it was made between God and the people of Israel for a particular purpose. They were to serve as witnesses to God's love for the people of the 'world' (and that may have meant those people groups amongst whom the People of Israel lived at the time, or might have meant the wider people groups of the world, through indirect transmission - such as trade). As we also know, the leaders of the people twisted this, deciding to tell the people that they had been chosen as a special nation, and to restrict the knowledge of this God to themselves.
The Old Testament record suggests that God suspected that this would happen, but chose to allow the people of Israel to make their own choices.
The result was that God's 'Plan B', as it were, had to be enacted which led to the good news of God's love for humanity being made available to all humanity through the 'New' Covenant.
Again, the people of the New Covenant haven't always served the purpose particularly well, but have also had their covenant hijacked by the likes of Constantine and his family; other authorities and even some power-greedy churchmen - such as the Borgias.
