I was once told by a GE tech that they are curved "because we can do that now".
Apparently, the use of CF for the core material not only allows for a very wide chord without the weight (and centrifugal force) penalty, the scimitar shape comes from advances in computer modeling of fluid dynamics.
Alongside of mass concerns, efficiency, noise, were among the primary design drivers with FOD core bypass for a bonus. RR, first to use wide-chord blades, are supposed to be working on their own CF fan blades.