[hylaride]

Because the plane would have certainly then required expensive/time consuming re-certification and expensive/time consuming (for the airlines) retraining of pilots, which Boeing was trying to avoid with the max. Keep in mind, the 737 originally flew in the late 60s. It's obviously now a very different aircraft, but if they were going to go fly-by-wire and require re-certification, they'd have designed a whole new aircraft (something a lot of people would have preferred even before the crashes).

The 373 max was a comparative rush job. When Airbus released the A320neo (the A320 first flew in the late 1980s so already was fly-by-wire), Boeing needed to get a comparable plane out ASAP or else cede several thousands of plane sales to Airbus. There wasn't the time to design a whole new plane, so they pushed the design of the 1960s era 373 as far as they could go. The details of the "risky" changes to accommodate the larger engines (that significantly changed the planes aerodynamic profile) and attempts to compensate for that are already documented elsewhere, but adding fly-by-wire would have only made things more complicated.