The law says that almost everything must be made clean by blood, and sins cannot be forgiven without blood to show death.
So the copies of the real things in heaven had to be made clean by animal sacrifices. But the real things in heaven need much better sacrifices.
Christ did not go into the Most Holy Place made by humans, which is only a copy of the real one. He went into heaven itself and is there now before God to help us.
The high priest enters the Most Holy Place once every year with blood that is not his own. But Christ did not offer himself many times.
Then he would have had to suffer many times since the world was made. But Christ came only once and for all time at just the right time to take away all sin by sacrificing himself.
Just as everyone must die once and be judged,
so Christ was offered as a sacrifice one time to take away the sins of many people. And he will come a second time, not to offer himself for sin, but to bring salvation to those who are waiting for him.