Because slaves inherently cost less than guests. Lower cost --> Lower production, Higher cost --> Higher production.
Getting a new slave essentially costs 60 caps/60 energy for the new cell, and then another 50 or so caps to buy one (or about 240 energy if you try to find one in the wild), so let's say 110 caps. A guest costs 100 caps/60 energy to build the room, then the 50 or so caps to buy a slave, then the energy/time upgrading them to the point that they won't flee when freed (let's say 200 energy = 40 caps), so a guest essentially costs 190 caps.
So this leaves the player with the option to:
Go the cheap route: focus mostly on slaves, since they cost less and produce less
Go the expensive route: focus mostly on guests, since they will maximize per capita production, while also having a fewer of them due to cost
Go the mixed route: have both in some sort of ratio that will depend on how they interact.
If you go the "guests only manage slaves", then that basically railroads players into the 3rd option. I think it's better to give the player more options on how they want to handle their workforce.
Though from my experiences, since money basically has no upper limit to how much you can earn in a day, but energy is capped at 120 per day, eventually you will be able to essentially get 1-2 slaves per day, but you can only upgrade a slave to guest once every 2 days or so. So I suspect every player will always have some slaves, simply because you can't upgrade them fast enough. Though that might change if other ways of building stats are added.
