The Death Busters were only found because of a direct result of Usagi's future self sending Chibi-Usa back in time on a mission to make friends, and Hotaru looked like an easy mark to Chibi. They had set up operations well before any senshi had ever been active which make you wonder why they didn't ever seem to fight Beryl's forces.
The initial rift that allowed Hotaru and her father to become possessed happened before Beryl. In the manga canon, though, the creatures from Pharoah 90's world couldn't survive for more than a couple of hours on Earth without a life support system. Most of the time before they started hunting the talismans was spent developing host bodies that wouldn't reject the possessing Daimon, and even by then they had only developed 5(the witches 5) who were not naturally compatible as Hotaru and her father were(either that or they were altered by the energies released in the explosion/Hotaru could survive because of being Sailor Saturn). As a result, they just weren't ready when Beryl was active.
An idea that I would like to see someone use is that it was that experiment which cracked the seal on Metallia in the first place. In the manga, the Queen Serenity AI on the moon activated the moon cats and had them activate the Senshi when the seal weakened enough for Metallia to call Beryl's reincarnation to it, awaken her past life, and start the process of escaping. Everyone involved could have been reincarnated several times in between, with the Queen's spell to send the Senshi to the future merely ensuring that they would be reborn whenever Beryl was, with a certain margin of error that caused them to be so much younger than her.