Scholars Perform Autopsy on Ancient Writing Systems
"When a system of writing begins to die, people probably don't even notice at first. Maybe the culture that spawned it loses its vitality, and the script decays along with it. Maybe the scribes or priests decide that most ordinary people aren't able to learn it, so they don't teach it."
"Or a new, simpler system may show up -- an alphabet, perhaps -- that can be easily learned by aggressive upstarts who don't speak the old language and don't care to learn its fancy pictographic forms."
FWIW, this is one reason why Bad English will someday defeat Correct French.