Why is there the Old Testament and New Testament?

This is because there was the time before Christ came to earth and which some of the rules at the Old Testament is followed by Jews and the rules in new testament in which Christians follow this was the time when Jesus came to earth to die for us. that is why Christians and Jews are two completely different religions, also the New Testament contains happenings from day to day life and relates to us and the future to come.
the old testament is about people getting ready for christ to come and the new testament is about christ.

AND


A testament is an agreement or covenant. God made a covenant with the nation of Israel.

Lev 26:12 I will walk among you and be your God, and you shall be My people.

This is the old covenant / testament

Because the nation of Israel turned away from God and failed to keep their part of the covenant, God made it possible for all people to have a relationship with him through His Son Jesus.

Heb 9:15 And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance.

This is the new covenant/ testament