Why are there two testaments?

The Bible is the record of the Hebrew people and the early Christians.  Thus, we have two kinds of testaments or covenants.  The historically older belief system of the people of Israel, was written from around 1,000 BCE to around 150 BCE.  On the other hand, the newer testament or covenant was written between 50 CE and 125 CE.  However, both testaments or covenants have a common base in the Hebrew Scripture.