The Borg has worked so well from the start for three reasons.
Fans of Star Trek know for sure who The Borg are. They are not really a race, not exactly, as they are a collective of different alien races who have been forced to join the Borg Collective. A collective that grows and expands through the galaxy by spreading their technological essence into the bodies of others, creating a drone from a once independent person.
That drone then becomes part of the collective, a mass collection of thoughts controlled in a singular thought to forever grow, expand and assimilate all that they meet. They’re essentially a science-fiction version of zombies if you want to simplify them.
Historically, they were never meant to exist. The Next Generation needed a new villain that wasn’t the Klingons and so they created The Ferengi. The Ferengi….well, they sucked as villains. They were small, diminutive, and looked more like Chernobyl-based gnomes than anything truly terrifying. So they became comedy relief characters and that seemed to work.
That meant that a new alien race had to be created for The Next Generation, and that allowed The Borg to take their place.
The Borg were introduced in season two of The Next Generation, and from the moment they were introduced, they started to make a sizeable impact on the franchise. Through all four of the Rick Berman/Brannan Braga era of Star Trek shows, The Borg appeared in every single franchise. They became a popular villain and that led to rating success.
Why did they work so well? Well, I found three components that make the Borg so wonderful of a villain.