I know how the date of Easter is calculated each year (the first Sunday after the first full moon after the Vernal Equinox). My question is, why is the date of Easter calculated this way?  Why isn't is a specific date, like December 25 for Christmas, used?  Or, to keep it on a Sunday, why isn't it a specific Sunday (like the third Sunday of April) used?

I realize that the Council of Nicaea decreed that this how the date for Easter be calculated, and I'm not questioning that, I'm just curious as to why this was chosen.