Why is the Solemnity of the Immaculate Conception treated differently?

I know that the Solemnity of the Immaculate Conception is a seemingly immoveable feast. Unlike the other holy days [the obligation is not transferred](http://catholicism.about.com/od/2013calendar/f/2013_Immaculate.htm) when it falls on a Saturday or a Monday (so you have to go to Mass two days in a row, and daily communicants whip out their tiny violins to play a festive round of Stabat Mater). But this year, the Immaculate Conception is on a Sunday, and the readings and everything are transferred to Monday but there is no addition obligation to attend Mass. So, my question is, what is so much more superlative about having the requirement to attend Mass abrogated every few years on one day for one feast and why is the Immaculate Conception more important than other Marian Feasts or All Saints Day (or Corpus Christi or Ascension Thursday, which are always transferred to Sundays in my diocese)? Is it only acceptable to celebrate a Christ/Trinity oriented feast day on a Sunday (Like Divine Mercy Sunday, Trinity Sunday or Christ the King of the Universe)?