Fairly easy:
First it was radio operators who gone.
Then navigators.
Then flight engineers.
If memory serves me right the last new non-russian aircraft type with FE entered service back in 1983 and this was 747-300. And the last all-new aircraft with FE was A300 back in 1974 - 40 (forty!) years ago.
So few decades past that milestone it is the right time to start changing regulations in order we can see pilotless commercial aircraft in 10 years from now.
Hopefully you don't feel that 50 years span between eliminating flight engineer and flight crew is too quick and things done in a rush?