Why is the F-117A the only plane to use the fully faceted design?
The F-117 was designed at a critical point in aeronautical history where we didn't have enough computing power to analyze the radar cross section of more complicated geometry, but did have the computing power to provide artificial stability to unstable designs.

The faceted design is not ideal for stealth or aerodynamics. Obviously the corners are less aerodynamic than a smooth surface. But it also means that if the facets align with a radar source they will reflect a strong return. The compromise is that it's easy to calculate the reflecting angles of radar energy off of flat surfaces based on various locations of radar sources relative to the aircraft.

A curved surface means that only a small section of the surface is really reflecting directly back to a radar source, while the rest is scattered. Together with radar absorbing materials, this can provide effective stealth. It just takes more analysis to determine how different curves and features will reflect radar energy.

You can see the evolution of stealth designs from the Have Blue (prototype for the F-117) in the mid 70's, to the Tacit Blue prototype in the late 70's, to the B-2 in the early 80's.