IIRC they swept because Hugo membership is open and they made a coordinated effort to join and sweep those categories. The idea of a coordinated anti-diversity effort pissed off a lot of people. For a few friends of mine who are lifelong sci-fi nerds, it was motivation to finally join Hugo so they could vote “no award”.
Amusingly (or enragingly) one of the effects of the Sad Puppy voting slate in 2015 was that it pushed William Patterson's magisterial and definitive biography of Robert A. Heinlein -- who the Puppies mostly adored -- off the shortlist for Best Related Word (a Hugo category usually occupied by scholarly works of SF history and criticism) in favour of utter garbage like "Wisdom From My Internet" by Michael Z. Williamson because the Sad Puppy organizers were so out of touch with events outside their bubble that they weren't aware the biography had been published.
You can take this as an illustration of the risk of an organized voting slate scoring a huge own goal.
They swept for of the same reason the Chinese worldcon was a mess: we are used to operating much of society by an unwritten set of rules that assume everyone is pretty much a good actor. Sure, they may have an axe to grind, and might stretch things, but no one will deliberately find loopholes at scale to abuse. This worked for several millenia. Whole countries used to not have written constitutions, just unwritten agreements on how to behave.
But in the age of the internet, it's much easier to find communities that you can break this way. These systems are resilient to a few people who are bad actors, but totally fall apart if there are coordinated actions.
It's exactly what the Republican party is doing in the US now. And what Trump supporters plan to do if he's elected with Project 2025.
