Empire has more players, period. That is the only real difference between the two factions.
More players, more premades, more healers, period. Bottom line, end of story.
When republic team with 2 or 3 healers is up against an imp team with only one healer, Imps get rolled as hard as anyone else.
Reality is that if imps really wanted more of a challenge, they would do more PUGs instead of premades for unranked WZs. But we all know the story there.