Warning: Spoilers ahead for The Book of Boba Fett episode 7!
The Book of Boba Fett ending served as a reminder of the brutality of the Clone Wars, when the Pykes unleashed two old Separatist droids on Mos Espa. It understandably left many viewers puzzled about why the Empire, which had no qualms about constructing planet-destroying superweapons, never used this kind of battle droid.
The Scorpenek Annihilator Droids numbered among the Separatists' rarest, most dangerous units - likely reflecting the sheer expense of constructing them. Their shields were resistant against most weapons-fire and even against lightsabers, while their firepower was sufficient to lay waste to entire platoons of Clone Troopers. During the Clone Wars, Scorpenek droids were often accompanied by several Droidekas, protected by their energy shields and making them even more dangerous. Given how effective the Scorpeneks were in The Book of Boba Fett, it is indeed surprising the Empire never made the most of this technology.
It's easy to underestimate the cultural impact the Clone Wars had on the galaxy, though. Anti-droid sentiment flared after the Clone Wars, with many blaming droids for the destruction and suffering caused during the conflict. When Palpatine established his Empire, he made a public show of destroying the Separatists' droids as part of his attempt to present himself as the galaxy's savior. This anti-droid sentiment became firmly entrenched as the decades ed, and by the time of the first Star Wars film, it wasn't unusual to see establishments like the Mos Eisley Cantina ban droids from entry. Palpatine still wanted the public to buy into Imperial propaganda, and using battle droids would have undermined this.
The Empire's military philosophy swiftly moved away from the Clone Wars. As seen in Project War-Mantle." This saw the Empire conscript untold numbers of soldiers from across the galaxy, training them to become a loyal army of stormtroopers. For Palpatine, this approach had the advantage of inducting millions of men and women into his Empire, ensuring they were bought into his propaganda, and he created a system where they would inevitably have to choose to corrupt themselves as they served the Empire, making them party to its crimes.
The Empire would depend upon its almost limitless supply of stormtroopers to win a battle, with the Emperor not caring how many died during an exercise because for every one stormtrooper lost there were another twenty in training. It's likely he also preferred the idea of having an army who were susceptible to the Force, rather than droids who couldn't be so easily influenced by it; it's notable that, while the Emperor did ultimately use Sentinel droids as personal agents, these Sentinel droids were powered by the dark side - suggesting he only turned to droids when he could corrupt them.
The Scorpenek droids in The Book of Boba Fett testify to the depth of the Pyke Syndicate's evil. When they unleashed these Clone Wars-era droids upon Mos Espa, they were declaring just how far they were willing to go to capture the planet - how many cultural taboos they would defy. Thus The Book of Boba Fett ended with the former bounty hunter celebrated as a hero for driving them back, even if his rancor did probably cause more damage to the town than the Scorpeneks.
