Chai, one thing that might have contributed to the new and varied accents is the difference in education.
I was born in 1943, (when dinosaurs still roamed the earth). Diction was addressed in all the schools I attended. In addition, one of my aunts was very like Noddy in her devotion to education. She would correct me whenever I mumbled or mispronounced a word. I did pronounce 'pin' and 'pen' differently (and properly). At the same time, I said nekkid rather than naked. Go figure?
TV and movies have had a profound influence on speech patterns and pronunciations. As for Vally Girls, yes, it was a fad for many years mainly, I believe, because of teen movies.
The one speech habit that I find most irritating is the rise at the end of a sentence. There are even news anchors who still have that mind numbing habit.
As a side note, I was born in California. I was six months old, when my parents moved to Texas My mother took me with her on the train. A lady in one of the facing seats was admiring me, saying what a pretty baby I was. She asked my mother where I was born. When my mother said Vallejo, California, the woman immediately responded, "Oh, I don't like people from California!"