Why do some people only need a few hours of sleep to feel energetic, while other require more?

Varies. All the experts tell us that everyone needs at least 8 hours of sleep for maximum health. Many get less, but at what cost ? Typically it does have negative effect over time. Check the caffiene intake of those people.
We are all different. The vast majority of people need a good 7-8 hrs to feel rested and energetic. Some need more and some can get away with less. Rarely can people get away with less than 6hrs and not have consequences. If our sleep is disturbed, we may need more to compensate. Some people tend to be energetic at baseline, and their sleepiness does not show. If you need more than ten hours, consult a physician.