"If this was such a beneficial mutation, why isn’t it more common?"
Based on natural selection it would only necessarily be more common if it affected reproduction. It may begin to affect reproduction now that people are sleeping less and those that don't cope well (like those that don't have the gene) may die before they reproduce; but until recently when sleep deprivation has become a larger issue, I doubt it ever affected reproduction enough for natural selection to do its job.
I think if people who need to sleep more is at a competitive disadvantage compared to other people, they will be less able to provide for the next generation, which will allow natural selection to slowly weed out those who don't have the gene.
Based on natural selection it would only necessarily be more common if it affected reproduction. It may begin to affect reproduction now that people are sleeping less and those that don't cope well (like those that don't have the gene) may die before they reproduce; but until recently when sleep deprivation has become a larger issue, I doubt it ever affected reproduction enough for natural selection to do its job.