Why do some people not believe in the resurrection of Jesus?

In the earliest manuscripts of Mark's Gospel, there was no mention of any appearances of the resurrected Jesus. The Gospel finished at verse 16:8, with the women fleeing in fear and telling no one that the tomb was empty. What is called the "Long Ending" (there was also a "Short Ending") was added much later by an interpolator.

Scholars now know that the authors of the other New Testament gospels relied on Mark's Gospel for everything they knew about the life and mission of Jesus. Yet each one described the risen Jesus as meeting the disciples at different times and places, with remarkably little consistency. It strains belief that evangelists knew nothing about the life of Jesus before his resurrection, copying what the knew from Mark, yet could tell us in detail about the resurrected Jesus. And it strains belief even more, that each account differs fundamentally from the others, yet all are true.

People who do not believe in the resurrection of Jesus do so because there is no credible evidence that it really happened.