Rarely is there an issue as contentious as taxation. Thick walls of partisan gridlock form around taxes like very few other issues. Reform is supposed to be a kind word and describe a way to make taxes simpler for people to understand, but the tax code actually sees changes on a regular annual basis or so. The problem is these changes haven't made things simpler for taxpayers.
True reform -- like a comprehensive restructuring of the tax code from the bottom up -- is the ideal goal, but realistically there are strong forces to maintain the status quo.