As Episcopalians (and many other Christians, too!), Easter isn't simply a day; it's a season. Called The Queen of Feasts, it lasts fifty days until the Day of Pentecost. Longer than any other season outside of Ordinary Time, Easter is the most joyous time of the year. The church is decked in white; the readings are all about resurrection and renewal, life and hope.
Easter is also full of Alleluias, the great shout of praise. Alleluia, or hallelujah in Hebrew, translates as "Praise ye the Lord!" There is always an implied exclamation point at the end, for we do not mumble Alleluia; we shout it. It returns to the fraction anthem in the Eucharist where it had been omitted during Lent, as well as taking its place in the opening acclamation - "Alleluia! Christ is risen! The Lord is risen indeed! Alleluia!"
In Easter - and only in Easter - Alleluia is added to the dismissal. Some parishes have adopted the custom (in direct violation of Prayer Book rubrics) of adding those Alleluias to the dismissal year round. I know some of our parishioners favor this practice as well. We don't do it, though, for the same reason we don't celebrate our birthday every day of the year. While we hopefully are thankful every day for our lives, and it's probably a safe bet that other people are happy we're around, too, we know that the celebration of our birth comes around once each year, on its anniversary. There is a different quality to our thanksgiving on that day, as there should be. Doing so marks that day as different from the rest of the year. The same applies for Easter. Yes, each Sunday is a feast of the Resurrection, and so in some sense each Sunday is a "Little Easter," but not every Sunday falls within Easter Season, that Queen of Feasts, and those that day deserve a little special treatment.