Typing. Leafeon has a total of 5 weaknesses to Fire, Ice, Bug, Flying and Poison.
Stats. Leafeon has 110 Attack, 130 Defense and 95 Speed, but those are its only redeeming qualities. It has 65 SpDef which means any good Special + Super Effective attack will kill it with its 65 HP.
Movepool. It doesn't have many physical moves to help its 110 Attack (basically only Leaf Blade), missing moves like Horn Leech and Power Whip. It also learns plenty of special moves with only 60 SpAtk, and can't cover all of its weaknesses.
And that's why people don't like Leafeon as much as Vaporeon or other Eeveelutions.