"Why did Jesus have to be sinless?"
Jesus came from heaven to deliver us from sin because only the sinless God-Man could break the power of sin for us.
Every human being is born with sin nature—born with the prideful inclination to rebel against our Creator and go our own way. The Bible calls this original sin, and it's a sickness we all inherited from our first parents, Adam and Eve. The problem is, from the beginning of time, man has tried and failed to make himself better. Why? Because sin has great power and therefore needs somebody who is far more powerful to break its grip.
So pure, righteous Jesus came from heaven to deliver us from sin because only the sinless God-Man could break the power of sin for us. Now you see why our culture makes such a grave error in denying the existence of sin.
Now you see why it breaks my heart that nearly half of professing evangelical Christians don't believe that Jesus was sinless. It's one of the devil's greatest deceptions. But anyone who acknowledges his sin and accepts Jesus' gift of salvation will be set free.
This is the real reason we rejoice at Christmastime: Because the sinless Christ came, gave His life, and rose again, the power of sin has been broken. He has set us free from the slavery of sin! And we will have the joy of being in His presence for all eternity. What Good News, indeed!