Wizards of the Coast eventually addressed the Standard imbalance through key bans, but the game was much more changed by the growing popularity of EDH (Elder Dragon Highlander), now known as Commander.
Many players began building decks around specific themes and incorporating roleplaying elements. These players often imposed self-restrictions to enhance the flavor of their decks.
Sure, you could add few Bat creatures to your Marrow-Gnawer deck — but a Wizard? Perhaps a grim warlock could join such a force, but definitely not a traditional wizard.
In this deckbuilding culture, Relentless Rats was thriving.
The flavor of swarming your opponent with a horde of rats is captivating.
The idea of an unstoppable pack growing stronger with each addition plays into a fantasy that many players enjoy. It's thematically satisfying and makes for fun, memorable games.
Another reason for Relentless Rats' popularity is its unique deck-building potential.
At first, the idea might seem odd.
Adding around 30 identical cards to your deck drastically limits space for other strategies and synergies. But it's precisely this approach that makes these decks truly unique.