Because they had all the time in the world to do this, and if they’d done it sooner, any errors would have cropped up early enough to correct, rather than your first notice being showing up on Election Day and being told “nope, don’t see you on the registrar.”
Mary Howard-Elley was erroneously de-registered in January and it took her until mid-October to prove she was a legal voter and get re-registered, and that was WITH the support of an investigative team to publicize the matter until election officials felt pressured to deal with it. If she’d been dropped in September, she’d have had no chance of fixing it in time. (PS: She was de-registered in the belief she was a non-citizen. She was born in Louisiana and is a lifelong Republican and Trump supporter. Leopards, faces…)
Along with Strod’s links, look up Mary Howard-Elley. If you object to the fact that she and at least nine other American citizens in Texas were nearly unable to vote on Tuesday, and are grateful that they are now able to do so thanks purely to ProPublica investigating the voter roll purges in Texas with deep skepticism and publicizing all the flaws in its execution, are you OK with voter fraud?
It sometimes helps to think of tolerance as a contract. If they break the contract, you can also break it, at least in tolerating their breaking of it. (Just don’t take it too far… you still need to tolerate even the most hostile bigot’s right to exist, you just don’t have to tolerate their bigoted words or actions.)
Some of the mice have reorganized as the Cats-Eating-Mice’s-Faces Party. (Don’t worry, surely the cat will only eat the faces of the OTHER mice.)