Take Kenessos, for example, who can put them directly into play for a reasonable mana investment
Suddenly you have seven goblins in play on turn 3, and a Muxus thatll swing in as a 12/12 before your opponent knows whats hit them

Zuri, Warrior of Wakanda Bast, Panther Goddess Okoye, Mighty and Adored Shuri, the Black Panther Storm, Queen of Wakanda T'Chaka, Venerable King Heart-Shaped Herb Kimoyo Beads N'Yami-Class Mother Ship Panther Habit Panther Robot Shuri's Fabricator Vibranium Mining Mech Vibranium Strike Gauntlets The Great Mound Divine Visitation Loyal Retainers Martial Coup Vanquish the Horde Birds of Paradise Conduit of Worlds Greater Good Nature's Lore Overwhelming Stampede Coveted Jewel Gilded Lotus Helm of the Host Metalwork Colossus Solemn Simulacrum Trading Post Bountiful Promenade Canopy Vista Fortified Village Razorverge Thicket Scattered Groves Scavenger Grounds Sungrass Prairie Sunpetal Grove Throne of the High City Scourglass Fleecemane Lion Hammer of Nazahn Mind's Eye Sword of the Animist Thran Dynamo Dispatch Generous Gift Ingenious Smith Palace Jailer Valorous Stance Beast Within Harmonize Loyal Guardian Arcane Signet Wakanda Meteor Golem Sol Ring Wakanda Whispersilk Cloak Command Tower Wakanda Evolving Wilds Path of Ancestry Terramorphic Expanse 12 Plains 12 Forest T'Challa, the Black Panther and his royal allies have arrived to pave a path to victory

While a majority of the cards in Historic Horizons will be reprints most of which are new to Arena, including many cards from both the first and second Modern Horizons sets the 31 brand new cards will be exclusive to Arenas digital platform
While that deck will certainly have pulled many copies of This Town Ain't Big Enough out of circulation, by itself it isn't the main reason for this spike
In my experience, once Ragost pulls this off once, it turns into a kill-on-sight Commander from that point on, at least in Bracket 3 settings