It takes one to be holy, sinless, flawless to negate sin, imperfection, and disobedience. Only God is holy and could do such a thing but God can't die. Jesus was in the form of God as He was with and part of God the Father in heaven. And, He was also human in that He was born of a woman, had feelings just like we do (felt pain, got tired, had to eat i/e was human) and could die. In this manner Jesus was God and man all rolled up into one. To denounce Jesus would also denounce the miracles he did like, walking on water, turning water into wine, feeding thousands with almost no resources, heal the deceased and sick by just touching them and He-Himself dieing but coming back to life. Jesus was prophesied of in the Old Testament and came to fruition in His birth=the New Testament.
Merely because He was crucified? MERELY? Read about crucifixion. It was the most painful, long lasting. shameful, inhumane torturous death at that time.
But, as a man Jesus could and did die. As God, Jesus could accept into His holy body the sins of all people for all time and forgive/negate them. That is exactly what He did. Then, Jesus rose from the dead, was seen by many people, and arose back to heaven. Our sins could now be erased and make us worthy and able to go to heaven. It takes acceptance, love, belief, and faith in God=Jesus to receive this. Read John 3:16 and Ephesians 2:5-10. to see what I mean. This is part of the reason that Christians believe as they do.