[Avodah] Why was arami chosen over more complete passages in shmos re: Exodus?

[Micha Berger]

On Tue, Apr 14, 2009 at 06:21:00PM -0400, rebshrink at aol.com wrote:
: I believe that Chazal carefully fulfilled the?chiyuv of "V'hegadta
> lvincha bayom hahu"by finding?a text that contains the formula (ie. the
> Cheftzah) of "V'hegadta". ? By Bikurim the Chumash tells us?that the
> bringer introduces?his statement of thanksgiving by "Hegadti hayom laShem
> Elokecha" and continues with four verses that describes the history of the
> Exodus? The Chumash thereby tells us that these four verses constitute a
> "hagadah"....


This just shifts the question. What constitutes a hagadah? Why does HQBH
use this label for Arami oveid avi to the exclusion of other tellings of
the story?

Tir'u baTov!
-Micha