>>58729220#1: It's a Cat/Dog/Fox, making it essentially the ideal "pet" pokemon. I even know several "normie" type people who nicknamed their eevee after their current dog/cat.
#2: It was a rare 1-per-game pokemon in the first generation, and the question of which evolution to get was an interesting debate, especially with trades being involved.
#3: While the original concept was designed by Motofumi Fujiwara, it was refined by Atsuko Nishida, who is essentially the best pokemon designer, responsible for pokemon like Pikachu, Dratini, Bulbasaur/Charmander/Squirtle, and of course, Munna. Eevee didn't really reach its current level of cuteness until its Yellow redesign.
#4: It's various evolutions have had roles in the battle meta, with Jolteon, Vaporeon, Umbreon, and Sylveon notably having key niches throughout the years.
#5: Incredibly popular with fan artists, because aside from being cute, has also been the subject of countless potential evolution designs.