>>38419475Three main reasons, plus one degenerate.
1, it's not gen 1, so it has none of that nostalgia
2, it's design is kinda generic, nothing really jumps out or appeals to a specific demographic, like Umbreon or Sylveon, and competitively it kinda blows
3, it released alongside Glaceon, which is one of the most beloved eevelutions.
Degenerately, it isn't particularly sexy-looking as a doggo, doesn't have any of them curves, and it barely counts as a plant for plantfuckers
>>38420541That's because Flareon has little purpose, lewdly. It's just bigger Eevee, so the only real context it has is if someone specifically wants a bigger, tougher Eevee, which is a niche within a niche