If 500 people saw Jesus ascend to heaven, why did only 4 of them write about it?

Because the bible clearly states that God was the one who inspired these 4 people to write about his appearing. That's just like if 500 people saw a car accident, of course the police wouldn't expect all 500 to testify as to what they saw, they're only going to require that the people who were nearest the accident report what they saw. Same thing!