That is a great question! Could the explanation be in the fact that Adam was human, but Satan is a spirit? Also, the WaterTower Bovine & Tractor Society would say that Adam's sin had nothing to do with divine soverignty but Satan's rebellion did raise this issue, so God had to let Satan continue to exist to settle that issue. (Hasn't it been settled yet? Seems to me human suffering and death could be halted at any time now, I wouldn't object). Any difficult question can be rational-lied by the WTB&TS.
Here's another question: The Israelites left Egypt with a great supply of Egyptian livestock. Yet in a short time they had no food and needer manna. What happened to all that livestock?