It's mostly because of one of the most attractive things about aviation (shared with the marine world to a large degree). Form and function, aerodynamic necessity and aesthetics tend to coincide. Aerodynamically efficient shapes also tend to be the most artistically pleasing shapes. Airplanes that fly well tend to look good.
Beyond that, you have the need to fit the radar antenna of whatever weather radar system you want to use, while achieving an efficient curvature and fineness ratio appropriate to the airplane's speed range.
Then you have to integrate the windshields into this. Flat windshields are way cheaper than curved ones, but are more draggy. And, going back to my original point, the curved ones just look way better. But 50 years ago a multi-laminate curved windshield that could handle 8 psi pressure differential would cost a fortune, so all the early airliners had flat windscreens and side panels.
It's a lot cheaper to make complicate curved windshields today so you see them pretty much on all new designs.