I was born in 1980, so I wasn't around for the 1970s obviously. My earliest fashion memories are neon/pastel colors and big permed hair. Anyway, why did the 1970s look so "dirty"? Peoples' hair and clothes just looked ... dirty. I know they weren't, but didn't people back then realize how they looked?
shareTo answer your question, just take a look at the 50s and 60s fashions. I think you'll get a pretty good idea how the whole thing evolved.
But maybe it has something to do with the filming itself...I know some 70s movies where everything looks dirty, the houses, the cars, the people...as I was born in '68, I remember a bit of the 70s, nothing was dirty, but the clothes and hair-dos were quite hilarious :-D