You are wrong. Other presidents have allowed Seders to take place in the White House, but they have not bothered to take part themselves. Obama is the first president to actually join in, and of his own accord, and that is not insignificant at all. The reason Rahm Emanuel is not mentioned is, in all likelihood, simply because, like most Jewish WH staffers, he prefers celebrating Passover with his family. So does David Axelrod, I am sure, but since he just got home from Afghanistan, and since his family is in Chicago, he will hardly have time to get to them.
U.S., allies conduct 1 air strike in Syria and 11 in Iraq against ISIS militants (Reuters)
from the article: Passover - an Obama family tradition