What is the obsession with Umbreon?

I understand the Eeveelutions are highly sought after, but Umbreon is in a class of its own. Growing up and playing the Gameboy games, Umbreon was never a demigod by any standards, not within the Eevee pedigree nor compared to its peers. On the other hand, Dragonite, Charizard, Gyarados, Celebi, and the legendary birds/dogs were the prized possessions in the first 251. Tyranitar and Houndoom are boss dark-types. So, what makes Umbreon stand out?