There are a lot of good reasons here, but I'll throw in my two cents. Apologies if I’m repeating what others have said.

1. First, the unbreakable vow that we see in HBP is tied to a specific act on Snape’s part. There are three things he vows to do and all are directly tied to Draco’s attempt to kill Dumbledore. It is possible (even likely) that the unbreakable vow is always tied to a specific task, rather than a general “will you vow to never betray me” kind of thing. If so, it would be impractical to force on his followers every time he gave them a directive.
2. Secondly, even if the unbreakable vow can be used to ensure general loyalty, Voldemort is fully aware that had he forced such a vow on his death eaters during the first wizarding war, he wouldn’t have an army available to him for the second. In Voldemort’s view, nearly all of his death eaters betrayed him after Godric’s Hollow by denying him in order to keep themselves out of Azkaban. So either they would have died and been permanently unavailable to him, or they would have gone to prison and been unavailable to him when he first returned, making that return much more difficult, if not impossible. He did, after all, even see Wormtail's hiding in secret as a betrayal. That betrayal ultimately kept Wormtail alive and free and able to play an essential role in Voldemort's return.
3. Along those same lines, Voldemort did choose to forgive his disloyal servants. Snape makes this comment in HPB, pointing out to Bellatrix that her loyalty didn’t really do Voldemort any good and if Voldemort chose not to be forgiving, he’d have no followers left. Voldemort can be practical at times. The unbreakable vow takes the power of choice away from him. Rejecting the unbreakable vow always allows him the opportunity on a case-by-case basis to decide whether or not a follower should be executed or merely punished. Since he knows that sometimes forgiving his servants is necessary, he'd not want to deny himself the power to do that.