The first real FBW system was probably the one put on the F16... to increase maneuverability, the aircraft was designed to be unstable in flight, so the FBW was necessary to keep the plane flyable. In that case, there was a compelling reason to develop FBW: a more maneuverable combat aircraft. Also, that system was ported to the F117, which had such atrocious aerodynamics that it, like the F16, was essentially unflyable without computer assistance.
In commercial aviation, the case is far less compelling. And it hasn't been an easy path. The very first A320 ended up landing in a forest, when it's flight computer mistook a slow flyby at an air show for a landing cycle. Oops... There have been several crashes traceable directly to FBW or sensor malfunctions (XL Airways/Air NZ acceptance flight, AOA sensor froze up) and Air Asia 8501, an A320 that crashed due to high altitude stall brought on by a malfunctioning rudder position sensor , to sensor malfunctions combined with different flight control layout, plus maybe fully automated aircraft lulling pilots into complacency (AF447, pitot tubes froze, bad sidestick design, inexperienced pilot), to just plain confusion in the cockpit working with FBW systems (Air Inter confusion setting descent rate, flew into a mountain).
Plus non fatal incidents that could easily have become fatal, like Qantas 72, where the inertial reference unit started sending weird data to the flight computer, sending the plane into a series of uncommanded dives and ascents.
FBW was supposed to make flying simpler, when in truth it just trades one set of complexities for another.
They have been slow in coming in commercial aviation because the advantages to airliners (who don't need more maneuverability or stealth capability) aren't nearly as compelling as they are on military aircraft, while the consequences of failure tend to kill a lot more people. If the FBW computer malfunctions in a military aircraft, the pilot punches out and has about an 85% chance of surviving. Airliners don't have that option.