Because the 737 already had a flight system, i.e. it is not a brand new aircraft. The ONLY brand new airplane Boeing builds in the 777. All others are newer versions of old airplanes. The 737-800 is virtually the same aircraft as the 35 year old 737-100. True, lots of electronics and computers have been added, but fly by wire it is not. It would cost too much; if you were going to spend the money to COMPLETELY redo EVERY control system and pretty much everything else, then just build a whole new airplane.
Same for the 764- it's just a long 763.