As far as I'm aware, use of FBW in the A320 onwards was for the following reasons:
1. To replace 'natural' pitch stability margins with 'active' control and thus to reduce trim drag, improving payload/range performance.
2. To remove the weight and complexity of 'traditional' mechanical control run (cables, pulleys, cranks etc) input devices to conventional powered flying control systems. This also makes maintenance simpler as everything associated with sidestick to surface is all-electric and provides self-diagnosis.
3. To give envelope protection to protect pilots from themselves at extremes of a/c attitude!
However, although the US has been using FBW in tactical a/c (e.g. F-16) for years, extension of the concept to airliners was viewed with much caution by Boeing et al. Their preferred option was to keep things simple and to use the tried and trusted traditional methods in their a/c. Until the 777, that is.
The side-stick is, I understand, something which virtually everyone who has converted to side-stick prefers just as soon as they become used to the fact that side stick deflexion controls manouevre demand rather than simply control surface angles.