Why has the Middle Class so declined?

[True Federalist (진정한 연방 주의자)]

I'd argue that the middle class hasn't declined at all.  Rather what happened is that for many years the material gap between the upper lower class and the lower middle class was miniscule or even inverted with the upper lower classes being better off than the lower middle classes materially.  That led to the perception that those upper lower classes were part of the middle class.  However the lower classes have steadily seen their material well being decline for many years now, leading to the restoration of the historical gap in material standards.  Simultaneously, a number of upper lower class jobs have been either exported to cheaper labor markets or replaced by smaller numbers of middle class jobs.