What killed the wishbone/flexbone?

There really doesn't seem to be a good place to learn some of the details of football strategy.
I know the wishbone dominated for years until something changed. What was it? Faster D players? A new defensive strategy that neutralized the wishbone? Rule changes?
I always assumed the "flex" in flexbone referred to the TBs going to the wing positions to be more 'flexible', i.e. much easier for them to be good passing targets past the LOS. If the flexbone defeated whatever killed the wishbone, what killed the flexbone?
I know the wishbone dominated for years until something changed. What was it? Faster D players? A new defensive strategy that neutralized the wishbone? Rule changes?
I always assumed the "flex" in flexbone referred to the TBs going to the wing positions to be more 'flexible', i.e. much easier for them to be good passing targets past the LOS. If the flexbone defeated whatever killed the wishbone, what killed the flexbone?