Mice need all the help they can get, lacking even the brief support most other tribes have
It answers all the premier threats of the format (Hullbreaker Horror, Esika's Chariot, all the Dragons, Adeline, Wrenn Treefolk) while also providing valuable counterplay to the creatureless Alrund's Epiphany and Dimir decks
When he deals combat damage to a player, every spell you cast from your hand for the rest of the turn gains cascade, creating a massive chain of value
Be reasonable
Horde of Notions Horde of Notions was, for a long time, the best option for a 5-color elemental commander
Both of them do a good job of showing up the expansions new mechanics and key features, and after playing them I dont think a third one would have been necessary, especially because it would have just been another Ruby/Emerald like last time