Whenever Eevee has parents, it's always Espeon and Umbreon.
Why can't Eevee have Glaceon and Leafeon as her parents?
Because both Glaceon and Leafeon has feminine attributes? Espeon has feminine attribute, Umbreon has masculine attributes; even if we know Glaceon and Leafeon can be both male and female most of people would assume the nearest gender with the visual; much like everyone assumed Gardevoir is female-only pokemon.