One reason could be that reduced sex drive is selected for at the level of the group, while higher sex drive is selected for at the level of the individual.
Assuming for the sake of discussion that this actually is an observable effect, I would expect it to arise because restricting the reproductive success of others is ancestrally adaptive for individuals, and one way of doing that when one is high-status in a group is by establishing/​supporting cultural norms that suppress sex by those of low status. It would follow that many groups would establish anti-sex norms to which the high-status elite were exempt.
Assuming for the sake of discussion that this actually is an observable effect, I would expect it to arise because restricting the reproductive success of others is ancestrally adaptive for individuals, and one way of doing that when one is high-status in a group is by establishing/​supporting cultural norms that suppress sex by those of low status. It would follow that many groups would establish anti-sex norms to which the high-status elite were exempt.