You may not see the reason for depopulation because you have no idea how the Elites think. They think of us as rats or roaches that need to be exterminated.
They do not.
We are a resource to be used.
If they exterminated everyone not an elite, who would be the labor force? No one.
Thus, you need labor.
Now 100 million slaves vs the, I'm guessing, 1,000 elites is not going to work out well. First, you need enforcers. People who are not slaves who can ensure slaves stay in line because if you use slaves as slave enforcers, you're basically giving guns to people who have no reason to listen to you.
Also the world is super big, so escaped slaves, people who aren't killed, existing infrastructure, etc... Are an issue.
Basically you need to secure 100 million people and build up the infrastruxture to keep them contained and working before you can enslave them.